PERLDIAG(1) Perl Programmers Reference Guide PERLDIAG(1)

perldiag - various Perl diagnostics

These messages are classified as follows (listed in increasing order of desperation):

    (W) A warning (optional).
    (D) A deprecation (enabled by default).
    (S) A severe warning (enabled by default).
    (F) A fatal error (trappable).
    (P) An internal error you should never see (trappable).
    (X) A very fatal error (nontrappable).
    (A) An alien error message (not generated by Perl).

The majority of messages from the first three classifications above (W, D & S) can be controlled using the "warnings" pragma.

If a message can be controlled by the "warnings" pragma, its warning category is included with the classification letter in the description below. E.g. "(W closed)" means a warning in the "closed" category.

Optional warnings are enabled by using the "warnings" pragma or the -w and -W switches. Warnings may be captured by setting $SIG{__WARN__} to a reference to a routine that will be called on each warning instead of printing it. See perlvar.

Severe warnings are always enabled, unless they are explicitly disabled with the "warnings" pragma or the -X switch.

Trappable errors may be trapped using the eval operator. See "eval" in perlfunc. In almost all cases, warnings may be selectively disabled or promoted to fatal errors using the "warnings" pragma. See warnings.

The messages are in alphabetical order, without regard to upper or lower-case. Some of these messages are generic. Spots that vary are denoted with a %s or other printf-style escape. These escapes are ignored by the alphabetical order, as are all characters other than letters. To look up your message, just ignore anything that is not a letter.

(W closed) You tried to do an accept on a closed socket. Did you forget to check the return value of your socket() call? See "accept" in perlfunc.
(S experimental::class) This warning is emitted if you use the "ADJUST" keyword of "use feature 'class'". This keyword is currently experimental and its behaviour may change in future releases of Perl.
(S experimental::refaliasing) This warning is emitted if you use a reference constructor on the left-hand side of an assignment to alias one variable to another. Simply suppress the warning if you want to use the feature, but know that in doing so you are taking the risk of using an experimental feature which may change or be removed in a future Perl version: 

    no warnings "experimental::refaliasing";
    use feature "refaliasing";
    \$x = \$y;
'%c' allowed only after types %s in %s
(F) The modifiers '!', '<' and '>' are allowed in pack() or unpack() only after certain types. See "pack" in perlfunc.
(W numeric) An alpha version can not be numified without losing information.
(W ambiguous) A subroutine you have declared has the same name as a Perl keyword, and you have used the name without qualification for calling one or the other. Perl decided to call the builtin because the subroutine is not imported.

To force interpretation as a subroutine call, either put an ampersand before the subroutine name, or qualify the name with its package. Alternatively, you can import the subroutine (or pretend that it's imported with the "use subs" pragma).

To silently interpret it as the Perl operator, use the "CORE::" prefix on the operator (e.g. CORE::log($x)) or declare the subroutine to be an object method (see "Subroutine Attributes" in perlsub or attributes).

(F) You wrote something like "tr/a-z-0//" which doesn't mean anything at all. To include a "-" character in a transliteration, put it either first or last. (In the past, "tr/a-z-0//" was synonymous with "tr/a-y//", which was probably not what you would have expected.)
(S ambiguous) You said something that may not be interpreted the way you thought. Normally it's pretty easy to disambiguate it by supplying a missing quote, operator, parenthesis pair or declaration.
(S ambiguous) You wrote something like "-foo", which might be the string "-foo", or a call to the function "foo", negated. If you meant the string, just write "-foo". If you meant the function call, write "-foo()".
(S ambiguous) "%", "&", and "*" are both infix operators (modulus, bitwise and, and multiplication) and initial special characters (denoting hashes, subroutines and typeglobs), and you said something like "*foo * foo" that might be interpreted as either of them. We assumed you meant the infix operator, but please try to make it more clear -- in the example given, you might write "*foo * foo()" if you really meant to multiply a glob by the result of calling a function.
(W ambiguous) You wrote something like "@{foo}", which might be asking for the variable @foo, or it might be calling a function named foo, and dereferencing it as an array reference. If you wanted the variable, you can just write @foo. If you wanted to call the function, write "@{foo()}" ... or you could just not have a variable and a function with the same name, and save yourself a lot of trouble.
(W ambiguous) You wrote something like "${foo[2]}" (where foo represents the name of a Perl keyword), which might be looking for element number 2 of the array named @foo, in which case please write $foo[2], or you might have meant to pass an anonymous arrayref to the function named foo, and then do a scalar deref on the value it returns. If you meant that, write "${foo([2])}".

In regular expressions, the "${foo[2]}" syntax is sometimes necessary to disambiguate between array subscripts and character classes. "/$length[2345]/", for instance, will be interpreted as $length followed by the character class "[2345]". If an array subscript is what you want, you can avoid the warning by changing "/${length[2345]}/" to the unsightly "/${\$length[2345]}/", by renaming your array to something that does not coincide with a built-in keyword, or by simply turning off warnings with "no warnings 'ambiguous';".

'|' and '<' may not both be specified on command line
(F) An error peculiar to VMS. Perl does its own command line redirection, and found that STDIN was a pipe, and that you also tried to redirect STDIN using '<'. Only one STDIN stream to a customer, please.
'|' and '>' may not both be specified on command line
(F) An error peculiar to VMS. Perl does its own command line redirection, and thinks you tried to redirect stdout both to a file and into a pipe to another command. You need to choose one or the other, though nothing's stopping you from piping into a program or Perl script which 'splits' output into two streams, such as 

    open(OUT,">$ARGV[0]") or die "Can't write to $ARGV[0]: $!";
    while (<STDIN>) {
        print;
        print OUT;
    }
    close OUT;
(W misc) The pattern match ("//"), substitution ("s///"), and transliteration ("tr///") operators work on scalar values. If you apply one of them to an array or a hash, it will convert the array or hash to a scalar value (the length of an array, or the population info of a hash) and then work on that scalar value. This is probably not what you meant to do. See "grep" in perlfunc and "map" in perlfunc for alternatives.
(F) msgsnd() requires a string at least as long as sizeof(long).
(W numeric) The indicated string was fed as an argument to an operator that expected a numeric value instead. If you're fortunate the message will identify which operator was so unfortunate.

Note that for the "Inf" and "NaN" (infinity and not-a-number) the definition of "numeric" is somewhat unusual: the strings themselves (like "Inf") are considered numeric, and anything following them is considered non-numeric.

(W layer) When pushing a layer with arguments onto the Perl I/O system you forgot the ) that closes the argument list. (Layers take care of transforming data between external and internal representations.) Perl stopped parsing the layer list at this point and did not attempt to push this layer. If your program didn't explicitly request the failing operation, it may be the result of the value of the environment variable PERLIO.
(W numeric) The indicated string was fed as an argument to the "++" operator which expects either a number or a string matching "/^[a-zA-Z]*[0-9]*\z/". See "Auto-increment and Auto-decrement" in perlop for details.
(W syntax) You called stat() on an array, but the array will be coerced to a scalar - the number of elements in the array.
(F) Each subroutine signature parameter declaration must start with a valid sigil; for example: 

    sub foo ($x, $, $y = 1, @z) {}
(F) Only scalar subroutine signature parameters may have a default value; for example: 

    sub foo ($x = 1)        {} # legal
    sub foo (@x = (1))      {} # invalid
    sub foo (%x = (a => b)) {} # invalid
(X) The malloc package that comes with Perl had an internal failure.
(X) A general assertion failed. The file in question must be examined.
(F) You tried to assign something that was not a reference to an lvalue reference (e.g., "\$x = $y"). If you meant to make $x an alias to $y, use "\$x = \$y".
(F) You tried to assign a reference to a reference constructor, but the two references were not of the same type. You cannot alias a scalar to an array, or an array to a hash; the two types must match. 

    \$x = \@y;  # error
    \@x = \%y;  # error
     $y = [];
    \$x = $y;   # error; did you mean \$y?
(F) When the "array_base" feature is disabled (e.g., and under "use v5.16;", and as of Perl 5.30) the special variable $[, which is deprecated, is now a fixed zero value.
(F) If you assign to a conditional operator, the 2nd and 3rd arguments must either both be scalars or both be lists. Otherwise Perl won't know which context to supply to the right side.
(W regexp) You had something like these: 

 [[:alnum]]
 [[:digit:xyz]

They look like they might have been meant to be the POSIX classes "[:alnum:]" or "[:digit:]". If so, they should be written:

 [[:alnum:]]
 [[:digit:]xyz]

Since these aren't legal POSIX class specifications, but are legal bracketed character classes, Perl treats them as the latter. In the first example, it matches the characters ":", "[", "a", "l", "m", "n", and "u".

If these weren't meant to be POSIX classes, this warning message is spurious, and can be suppressed by reordering things, such as

 [[al:num]]

or

 [[:munla]]
<> at require-statement should be quotes
(F) You wrote "require <file>" when you should have written "require 'file'".
(F) The failing code has attempted to get or set a key which is not in the current set of allowed keys of a restricted hash.
(F) You wrote "bless $foo" with one argument after somehow causing the current package to be freed. Perl cannot figure out what to do, so it throws up its hands in despair.
(F) You are attempting to call "bless" with a package name that is a new-style "class". This is not necessary, as instances created by the constructor are already in the correct class. Instances cannot be created by other means, such as "bless".
(F) The CLASSNAME argument to the bless() operator is expected to be the name of the package to bless the resulting object into. You've supplied instead a reference to something: perhaps you wrote 

    bless $self, $proto;

when you intended

    bless $self, ref($proto) || $proto;

If you actually want to bless into the stringified version of the reference supplied, you need to stringify it yourself, for example by:

    bless $self, "$proto";
(S debugging) An array was assigned to when it was being freed. Freed values are not supposed to be visible to Perl code. This can also happen if XS code calls "av_clear" from a custom magic callback on the array.
(F) The failing code attempted to delete from a restricted hash a key which is not in its key set.
(F) The failing code attempted to delete a key whose value has been declared readonly from a restricted hash.
(S internal) All SV objects are supposed to be allocated from arenas that will be garbage collected on exit. An SV was discovered to be outside any of those arenas.
(S internal) Perl maintains a reference-counted internal table of strings to optimize the storage and access of hash keys and other strings. This indicates someone tried to decrement the reference count of a string that can no longer be found in the table.
(S debugging) Mortalized values are supposed to be freed by the free_tmps() routine. This indicates that something else is freeing the SV before the free_tmps() routine gets a chance, which means that the free_tmps() routine will be freeing an unreferenced scalar when it does try to free it.
(S internal) The reference counts got screwed up on symbol aliases.
(S internal) Perl went to decrement the reference count of a scalar to see if it would go to 0, and discovered that it had already gone to 0 earlier, and should have been freed, and in fact, probably was freed. This could indicate that SvREFCNT_dec() was called too many times, or that SvREFCNT_inc() was called too few times, or that the SV was mortalized when it shouldn't have been, or that memory has been corrupted.
(W pack) You tried to pass a temporary value (like the result of a function, or a computed expression) to the "p" pack() template. This means the result contains a pointer to a location that could become invalid anytime, even before the end of the current statement. Use literals or global values as arguments to the "p" pack() template to avoid this warning.
(F) You tried to load a file with "use" or "require" that failed to compile once already. Perl will not try to compile this file again unless you delete its entry from %INC. See "require" in perlfunc and "%INC" in perlvar.
(W misc) You tried to set the length of an array which has been freed. You can do this by storing a reference to the scalar representing the last index of an array and later assigning through that reference. For example 

    $r = do {my @a; \$#a};
    $$r = 503
(W substr) You supplied a reference as the first argument to substr() used as an lvalue, which is pretty strange. Perhaps you forgot to dereference it first. See "substr" in perlfunc.
(W misc) A sub was declared as sub foo : prototype(A) : prototype(B) {}, for example. Since each sub can only have one prototype, the earlier declaration(s) are discarded while the last one is applied.
(S debugging) This indicates that something went wrong and Perl got very confused about @_ or @DB::args being tied.
(F) You passed a buffer of the wrong size to one of msgctl(), semctl() or shmctl(). In C parlance, the correct sizes are, respectively, sizeof(struct msqid_ds *), sizeof(struct semid_ds *), and sizeof(struct shmid_ds *).
(F) You've used the "/e" switch to evaluate the replacement for a substitution, but perl found a syntax error in the code to evaluate, most likely an unexpected right brace '}'.
(F) A symbol was passed to something wanting a filehandle, but the symbol has no filehandle associated with it. Perhaps you didn't do an open(), or did it in another package.
(S malloc) An internal routine called free() on something that had never been malloc()ed in the first place. Mandatory, but can be disabled by setting environment variable "PERL_BADFREE" to 0.

This message can be seen quite often with DB_File on systems with "hard" dynamic linking, like "AIX" and "OS/2". It is a bug of "Berkeley DB" which is left unnoticed if "DB" uses forgiving system malloc().

(F) A plugin using the "PL_infix_plugin" mechanism to parse an infix keyword consumed part of a named identifier operator name but did not consume all of it. This is not permitted as it leads to fragile parsing results.
(A) You've accidentally run your script through csh instead of Perl. Check the #! line, or manually feed your script into Perl yourself.
(F) You started to name a symbol by using a package prefix, and then didn't finish the symbol. In particular, you can't interpolate outside of quotes, so 

    $var = 'myvar';
    $sym = mypack::$var;

is not the same as

    $var = 'myvar';
    $sym = "mypack::$var";
(F) An extension using the keyword plugin mechanism violated the plugin API.
(S malloc) An internal routine called realloc() on something that had never been malloc()ed in the first place. Mandatory, but can be disabled by setting the environment variable "PERL_BADFREE" to 1.
(P) An internal request asked to add an entry of the named type to something that wasn't a symbol table entry.
(P) An internal request asked to add a scalar entry to something that wasn't a symbol table entry.
(W bareword) The compiler found a bareword where it expected a conditional, which often indicates that an || or && was parsed as part of the last argument of the previous construct, for example: 

    open FOO || die;

It may also indicate a misspelled constant that has been interpreted as a bareword:

    use constant TYPO => 1;
    if (TYOP) { print "foo" }

The "strict" pragma is useful in avoiding such errors.

(F) The bareword form of require has been invoked with a filename which could not have been generated by a valid bareword permitted by the parser. You shouldn't be able to get this error from Perl code, but XS code may throw it if it passes an invalid module name to "Perl_load_module".
(F) In "require Bare::Word", the bareword is not allowed to start with a double-colon. Write "require ::Foo::Bar" as "require Foo::Bar" instead.
(F) With "strict subs" in use, a bareword is only allowed as a subroutine identifier, in curly brackets or to the left of the "=>" symbol. Perhaps you need to predeclare a subroutine?
(W bareword) You used a qualified bareword of the form "Foo::", but the compiler saw no other uses of that namespace before that point. Perhaps you need to predeclare a package?
(F) You attempted to use a bareword filehandle with the "bareword_filehandles" feature disabled.

Only the built-in handles "STDIN", "STDOUT", "STDERR", "ARGV", "ARGVOUT" and "DATA" can be used with the "bareword_filehandles" feature disabled.

(F) An untrapped exception was raised while executing a BEGIN subroutine. Compilation stops immediately and the interpreter is exited.
(F) Perl found a "BEGIN {}" subroutine (or a "use" directive, which implies a "BEGIN {}") after one or more compilation errors had already occurred. Since the intended environment for the "BEGIN {}" could not be guaranteed (due to the errors), and since subsequent code likely depends on its correct operation, Perl just gave up.
\%d better written as $%d
(W syntax) Outside of patterns, backreferences live on as variables. The use of backslashes is grandfathered on the right-hand side of a substitution, but stylistically it's better to use the variable form because other Perl programmers will expect it, and it works better if there are more than 9 backreferences.
(W portable) The binary number you specified is larger than 2**32-1 (4294967295) and therefore non-portable between systems. See perlport for more on portability concerns.
(W closed) You tried to do a bind on a closed socket. Did you forget to check the return value of your socket() call? See "bind" in perlfunc.
(W unopened) You tried binmode() on a filehandle that was never opened. Check your control flow and number of arguments.
(W portable) Using bit vector sizes larger than 32 is non-portable.
(P) Perl detected an attempt to copy an internal value that is not copiable.
(P) When starting a new thread or returning values from a thread, Perl encountered an invalid data type.
(W regexp) (only under "use re 'strict'" or within "(?[...])")

In a bracketed character class in a regular expression pattern, you had a range which has exactly one end of it specified using "\N{}", and the other end is specified using a non-portable mechanism. Perl treats the range as a Unicode range, that is, all the characters in it are considered to be the Unicode characters, and which may be different code points on some platforms Perl runs on. For example, "[\N{U+06}-\x08]" is treated as if you had instead said "[\N{U+06}-\N{U+08}]", that is it matches the characters whose code points in Unicode are 6, 7, and 8. But that "\x08" might indicate that you meant something different, so the warning gets raised.

(W internal) A warning peculiar to VMS. While Perl was preparing to iterate over %ENV, it encountered a logical name or symbol definition which was too long, so it was truncated to the string shown.
(S experimental::builtin) A call is being made to a function in the "builtin::" namespace, which is currently experimental. The existence or nature of the function may be subject to change in a future version of Perl.
(F) The "import" method of the "builtin" package was invoked when no code is currently being compiled. Since this method is used to introduce new lexical subroutines into the scope currently being compiled, this is not going to have any effect.
(F) You attempted to "use builtin :ver" for a version number that is either older than 5.39 (when the ability was added), or newer than the current perl version.
(F) A subroutine invoked from an external package via call_sv() exited by calling exit.
%s() called too early to check prototype
(W prototype) You've called a function that has a prototype before the parser saw a definition or declaration for it, and Perl could not check that the call conforms to the prototype. You need to either add an early prototype declaration for the subroutine in question, or move the subroutine definition ahead of the call to get proper prototype checking. Alternatively, if you are certain that you're calling the function correctly, you may put an ampersand before the name to avoid the warning. See perlsub.
(F) An attempt was made to apply a parameter name to a field, when the name is already being used by another field in the same class, or one of its parent classes. This would cause a name clash so is not allowed.
(F) You passed an invalid number (like an infinity or not-a-number) to "chr".
(F) Your perl script appears to have changed directory while performing an in-place edit of a file specified by a relative path, and your system doesn't include the directory relative POSIX functions needed to handle that.
(F) You tried compressing an infinity or not-a-number as an unsigned integer with BER, which makes no sense.
(F) An argument to pack("w",...) was too large to compress. The BER compressed integer format can only be used with positive integers, and you attempted to compress a very large number (> 1e308). See "pack" in perlfunc.
(F) An argument to pack("w",...) was negative. The BER compressed integer format can only be used with positive integers. See "pack" in perlfunc.
(F) You manipulated Perl's symbol table directly, stored a reference in it, then tried to access that symbol via conventional Perl syntax. The access triggers Perl to autovivify that typeglob, but it there is no legal conversion from that type of reference to a typeglob.
(P) Perl detected an attempt to copy a value to an internal type that cannot be directly assigned to.
(F) An attempt was made to create a class out of a package that already has an @ISA array, and the array is not empty. This is not permitted, as it would lead to a class with inconsistent inheritance.
(F) An attempt was made to create an object of a class where the start of the class definition has been seen, but the class has not been completed.

This can happen for a failed eval, or if you attempt to create an object at compile time before the class is complete:

  eval "class Foo {"; Foo->new; # error
  class Bar { BEGIN { Bar->new } }; # error
(S io) You tried to apply an encoding that did not exist to a filehandle, either with open() or binmode().
(F) You tried to directly call a "method" subroutine of one class by passing in a value that is an instance of a different class. This is not permitted, as the method would not have access to the correct instance fields.
(F) You tried to directly call a "method" subroutine of a class by passing in a value that is not an instance of that class. This is not permitted, as the method would not then have access to its instance fields.
(F) You tried to use opendir() to associate a dirhandle to a symbol (glob or scalar) that already holds a filehandle. Since this idiom might render your code confusing, it was deprecated in Perl 5.10. As of Perl 5.28, it is a fatal error.
(F) You tried to use open() to associate a filehandle to a symbol (glob or scalar) that already holds a dirhandle. Since this idiom might render your code confusing, it was deprecated in Perl 5.10. As of Perl 5.28, it is a fatal error.
(F) You attempted to use one of the keywords that only makes sense inside a "class" definition, at a location that is not inside such a class.
(F) You tried converting an infinity or not-a-number to an integer, which makes no sense.
(F) You tried printing an infinity or not-a-number as a character (%c), which makes no sense. Maybe you meant '%s', or just stringifying it?
(F) You tried to begin a "class" definition for a class that already exists. A class may only have one definition block.
(F) "caller" tried to set @DB::args, but found it tied. Tying @DB::args is not supported. (Before this error was added, it used to crash.)
(P) You somehow managed to call "tie" on an array that does not keep a reference count on its arguments and cannot be made to do so. Such arrays are not even supposed to be accessible to Perl code, but are only used internally.
(F) A "__CLASS__" expression yields the class name of the object instance executing the current method, and therefore it can only be placed inside an actual method (or method-like expression, such as a field initializer expression).
(F) Some XS code tried to use sv_vcatpvfn() or a related function with a format string that specifies explicit indexes for some of the elements, and using a C-style variable-argument list (a "va_list"). This is not currently supported. XS authors wanting to do this must instead construct a C array of "SV*" scalars containing the arguments.
(F) An argument to pack("w",...) was not an integer. The BER compressed integer format can only be used with positive integers, and you attempted to compress something else. See "pack" in perlfunc.
(F) An attempt was made to jump out of the scope of a "defer" block by using a control-flow statement such as "return", "goto" or a loop control. This is not permitted.
(F) Similar to above, but involving a "finally" block at the end of a "try"/"catch" construction rather than a "defer" block.
(F) You attempted to call "bless" on a value that already refers to a real object instance.
(F) Only hard references may be blessed. This is how Perl "enforces" encapsulation of objects. See perlobj.
(F) You called "break", but you're in a "foreach" block rather than a "given" block. You probably meant to use "next" or "last".
(F) You called "break", but you're not inside a "given" block.
(S) This should not happen. Internals code has set up a destructor using "mortal_destructor_sv" or "mortal_destructor_x" which is firing during global destruction. Please attempt to reduce the code that triggers this warning down to a small an example as possible and then report the problem to <https://github.com/Perl/perl5/issues/new/choose>
(F) You used the syntax of a method call, but the slot filled by the object reference or package name contains an undefined value. Something like this will reproduce the error: 

    $BADREF = undef;
    process $BADREF 1,2,3;
    $BADREF->process(1,2,3);
(F) A method call must know in what package it's supposed to run. It ordinarily finds this out from the object reference you supply, but you didn't supply an object reference in this case. A reference isn't an object reference until it has been blessed. See perlobj.
(F) You used the syntax of a method call, but the slot filled by the object reference or package name contains an expression that returns a defined value which is neither an object reference nor a package name. Something like this will reproduce the error: 

    $BADREF = 42;
    process $BADREF 1,2,3;
    $BADREF->process(1,2,3);
(P) Perl got confused as to whether a hash was a plain hash or a symbol table hash when trying to update @ISA caches.
(F) An XS module tried to call "mro_method_changed_in" on a hash that was not attached to the symbol table.
(F) You called "perl -x/foo/bar", but /foo/bar is not a directory that you can chdir to, possibly because it doesn't exist.
(F) Certain types of SVs, in particular real symbol table entries (typeglobs), can't be forced to stop being what they are. So you can't say things like: 

    *foo += 1;

You CAN say

    $foo = *foo;
    $foo += 1;

but then $foo no longer contains a glob.

(F) You called "continue", but you're not inside a "when" or "default" block.
(F) On Cygwin, you called a path conversion function with an empty path. Only non-empty paths are legal.
(P) An error peculiar to VMS. The process is suffering from exhausted quotas or other plumbing problems.
(F) Only scalar, array, and hash variables may be declared as "my", "our" or "state" variables. They must have ordinary identifiers as names.
(F) You have used a "default" block that is neither inside a "foreach" loop nor a "given" block. (Note that this error is issued on exit from the "default" block, so you won't get the error if you use an explicit "continue".)
(S) This warning indicates something wrong in the internals of perl. Perl was trying to find the class (e.g. LISTOP) of a particular OP, and was unable to do so. This is likely to be due to a bug in the perl internals, or due to a bug in XS code which manipulates perl optrees.
(S inplace) You tried to use the -i switch on a special file, such as a file in /dev, a FIFO or an uneditable directory. The file was ignored.
(S inplace) The creation of the new file failed for the indicated reason.
(S inplace) Your filesystem does not support filenames longer than 14 characters and Perl was unable to create a unique filename during inplace editing with the -i switch. The file was ignored.
(W locale) You are 1) running under ""use locale""; 2) the current locale is not a UTF-8 one; 3) you tried to do the designated case-change operation on the specified Unicode character; and 4) the result of this operation would mix Unicode and locale rules, which likely conflict. Mixing of different rule types is forbidden, so the operation was not done; instead the result is the indicated value, which is the best available that uses entirely Unicode rules. That turns out to almost always be the original character, unchanged.

It is generally a bad idea to mix non-UTF-8 locales and Unicode, and this issue is one of the reasons why. This warning is raised when Unicode rules would normally cause the result of this operation to contain a character that is in the range specified by the locale, 0..255, and hence is subject to the locale's rules, not Unicode's.

If you are using locale purely for its characteristics related to things like its numeric and time formatting (and not "LC_CTYPE"), consider using a restricted form of the locale pragma (see "The "use locale" pragma" in perllocale) like ""use locale ':not_characters'"".

Note that failed case-changing operations done as a result of case-insensitive "/i" regular expression matching will show up in this warning as having the "fc" operation (as that is what the regular expression engine calls behind the scenes.)

(F) This machine doesn't have either waitpid() or wait4(), so only waitpid() without flags is emulated.
(F) The #! line specifies a switch that doesn't make sense at this point. For example, it'd be kind of silly to put a -x on the #! line.
(F) Your platform's byte-order is neither big-endian nor little-endian, or it has a very strange pointer size. Packing and unpacking big- or little-endian floating point values and pointers may not be possible. See "pack" in perlfunc.
(W exec) A system(), exec(), or piped open call could not execute the named program for the indicated reason. Typical reasons include: the permissions were wrong on the file, the file wasn't found in $ENV{PATH}, the executable in question was compiled for another architecture, or the #! line in a script points to an interpreter that can't be run for similar reasons. (Or maybe your system doesn't support #! at all.)
(F) Perl was trying to execute the indicated program for you because that's what the #! line said. If that's not what you wanted, you may need to mention "perl" on the #! line somewhere.
(F) You used the -S switch, but the copies of the script to execute found in the PATH did not have correct permissions.
(F) A string of a form "CORE::word" was given to prototype(), but there is no builtin with the name "word".
(F) You said to goto a label that isn't mentioned anywhere that it's possible for us to go to. See "goto" in perlfunc.
(F) You used the -S switch, but the script to execute could not be found in the PATH.
(F) You used the -S switch, but the script to execute could not be found in the PATH, or at least not with the correct permissions. The script exists in the current directory, but PATH prohibits running it.
(F) Perl strings can stretch over multiple lines. This message means that the closing delimiter was omitted. Because bracketed quotes count nesting levels, the following is missing its final parenthesis: 

    print q(The character '(' starts a side comment.);

If you're getting this error from a here-document, you may have included unseen whitespace before or after your closing tag or there may not be a linebreak after it. A good programmer's editor will have a way to help you find these characters (or lack of characters). See perlop for the full details on here-documents.

(F) The named property which you specified via "\p" or "\P" is not one known to Perl. Perhaps you misspelled the name? See "Properties accessible through \p{} and \P{}" in perluniprops for a complete list of available official properties. If it is a user-defined property it must have been defined by the time the regular expression is matched.

If you didn't mean to use a Unicode property, escape the "\p", either by "\\p" (just the "\p") or by "\Q\p" (the rest of the string, or until "\E").

(F) A fatal error occurred while trying to fork while opening a pipeline.
(W pipe) A fork in a piped open failed with EAGAIN and will be retried after five seconds.
(S) A warning peculiar to VMS. This arises because of the difference between access checks under VMS and under the Unix model Perl assumes. Under VMS, access checks are done by filename, rather than by bits in the stat buffer, so that ACLs and other protections can be taken into account. Unfortunately, Perl assumes that the stat buffer contains all the necessary information, and passes it, instead of the filespec, to the access-checking routine. It will try to retrieve the filespec using the device name and FID present in the stat buffer, but this works only if you haven't made a subsequent call to the CRTL stat() routine, because the device name is overwritten with each call. If this warning appears, the name lookup failed, and the access-checking routine gave up and returned FALSE, just to be conservative. (Note: The access-checking routine knows about the Perl "stat" operator and file tests, so you shouldn't ever see this warning in response to a Perl command; it arises only if some internal code takes stat buffers lightly.)
(P) An error peculiar to VMS. After creating a mailbox to act as a pipe, Perl can't retrieve its name for later use.
(P) An error peculiar to VMS. Perl asked $GETSYI how big you want your mailbox buffers to be, and didn't get an answer.
(F) A "goto" statement was executed to jump into the middle of a binary or list expression. You can't get there from here. The reason for this restriction is that the interpreter would get confused as to how many arguments there are, resulting in stack corruption or crashes. This error occurs in cases such as these: 

    goto F;
    print do { F: }; # Can't jump into the arguments to print
    goto G;
    $x + do { G: $y }; # How is + supposed to get its first operand?
(F) A "goto" statement was executed to jump into the scope of a "defer" block. This is not permitted.
(F) A "goto" statement was executed to jump into the middle of a "given" block. You can't get there from here. See "goto" in perlfunc.
(F) A "goto" statement was executed to jump into the middle of a foreach loop. You can't get there from here. See "goto" in perlfunc.
(F) A "goto" statement was executed to jump out of what might look like a block, except that it isn't a proper block. This usually occurs if you tried to jump out of a sort() block or subroutine, which is a no-no. See "goto" in perlfunc.
(F) The "goto subroutine" call can't be used to jump out of an eval "string" or block.
(F) The "goto subroutine" call can't be used to jump out of the comparison sub for a sort(), or from a similar callback (such as the reduce() function in List::Util).
(F) The deeply magical "goto subroutine" call can only replace one subroutine call for another. It can't manufacture one out of whole cloth. In general you should be calling it out of only an AUTOLOAD routine anyway. See "goto" in perlfunc.
(W signal) Perl has detected that it is being run with the SIGCHLD signal (sometimes known as SIGCLD) disabled. Since disabling this signal will interfere with proper determination of exit status of child processes, Perl has reset the signal to its default value. This situation typically indicates that the parent program under which Perl may be running (e.g. cron) is being very careless.
(F) Process identifiers must be (signed) integers. It is a fatal error to attempt to kill() an undefined, empty-string or otherwise non-numeric process identifier.
(F) A "last" statement was executed to break out of the current block, except that there's this itty bitty problem called there isn't a current block. Note that an "if" or "else" block doesn't count as a "loopish" block, as doesn't a block given to sort(), map() or grep(). You can usually double the curlies to get the same effect though, because the inner curlies will be considered a block that loops once. See "last" in perlfunc.
(F) Perl tried to calculate the method resolution order (MRO) of a package, but failed because the package stash has no name.
(F) The module you tried to load failed to load a dynamic extension. This may either mean that you upgraded your version of perl to one that is incompatible with your old dynamic extensions (which is known to happen between major versions of perl), or (more likely) that your dynamic extension was built against an older version of the library that is installed on your system. You may need to rebuild your old dynamic extensions.
(F) You used local on a variable name that was previously declared as a lexical variable using "my" or "state". This is not allowed. If you want to localize a package variable of the same name, qualify it with the package name.
(F) You said something like "local $$ref", which Perl can't currently handle, because when it goes to restore the old value of whatever $ref pointed to after the scope of the local() is finished, it can't be sure that $ref will still be a reference.
(F) You said to "do" (or "require", or "use") a file that couldn't be found. Perl looks for the file in all the locations mentioned in @INC, unless the file name included the full path to the file. Perhaps you need to set the PERL5LIB or PERL5OPT environment variable to say where the extra library is, or maybe the script needs to add the library name to @INC. Or maybe you just misspelled the name of the file. See "require" in perlfunc and lib.
(F) A function (or method) was called in a package which allows autoload, but there is no function to autoload. Most probable causes are a misprint in a function/method name or a failure to "AutoSplit" the file, say, by doing "make install".
(F) The module you loaded is trying to load an external library, like for example, foo.so or bar.dll, but the DynaLoader module was unable to locate this library. See DynaLoader.
(F) You called a method correctly, and it correctly indicated a package functioning as a class, but that package doesn't define that particular method, nor does any of its base classes. See perlobj.
(F) You called a method on a class that did not exist, and the method could not be found in UNIVERSAL. This often means that a method requires a package that has not been loaded.
(F) You pushed an object, either directly or via an array reference hook, into @INC, but the object doesn't support any known hook methods, nor a string overload and is also not a blessed CODE reference. In short the "require" function does not know what to do with the object. See also "require" in perlfunc.
(D deprecated::missing_import_called_with_args) You called the import() or unimport() method of a class that has no import method defined in its inheritance graph, and passed an argument to the method. This is very often the sign of a mispelled package name in a use or require statement that has silently succeded due to a case insensitive file system.

Another common reason this may happen is when mistakenly attempting to import or unimport a symbol from a class definition or package which does not use "Exporter" or otherwise define its own "import" or "unimport" method.

(W syntax) The @ISA array contained the name of another package that doesn't seem to exist.
(F) You tried to use in open() a PerlIO layer that does not exist, e.g. open(FH, ">:nosuchlayer", "somefile").
(F) List assignment to %ENV is not supported on some systems, notably VMS.
(S) A module passed the flag 0x01 to DynaLoader::dl_load_file() to request that symbols from the stated file are made available globally within the process, but that functionality is not available on this platform. Whilst the module likely will still work, this may prevent the perl interpreter from loading other XS-based extensions which need to link directly to functions defined in the C or XS code in the stated file.
(F) You aren't allowed to assign to the item indicated, or otherwise try to change it, such as with an auto-increment.
(F) Subroutines meant to be used in lvalue context should be declared as such. See "Lvalue subroutines" in perlsub.
(F) Only a limited number of constructs can be used as the argument to a reference constructor on the left-hand side of an assignment, and what you used was not one of them. See "Assigning to References" in perlref.
(F) Assigning to "\local(@array)" or "\(local @array)" is not supported, as it is not clear exactly what it should do. If you meant to make @array refer to some other array, use "\@array = \@other_array". If you want to make the elements of @array aliases of the scalars referenced on the right-hand side, use "\(@array) = @scalar_refs".
(F) Assigning to "\(%hash)" is not supported. If you meant to make %hash refer to some other hash, use "\%hash = \%other_hash". If you want to make the elements of %hash into aliases of the scalars referenced on the right-hand side, use a hash slice: "\@hash{@keys} = @those_scalar_refs".
(F) The target of a msgrcv must be modifiable to be used as a receive buffer.
(F) A "next" statement was executed to reiterate the current block, but there isn't a current block. Note that an "if" or "else" block doesn't count as a "loopish" block, as doesn't a block given to sort(), map() or grep(). You can usually double the curlies to get the same effect though, because the inner curlies will be considered a block that loops once. See "next" in perlfunc.
(S inplace) The implicit opening of a file through use of the "<>" filehandle, either implicitly under the "-n" or "-p" command-line switches, or explicitly, failed for the indicated reason. Usually this is because you don't have read permission for a file which you named on the command line.

(F) You tried to call perl with the -e switch, but /dev/null (or your operating system's equivalent) could not be opened.

(W io) You tried to open a scalar reference for reading or writing, using the 3-arg open() syntax: 

    open FH, '>', $ref;

but your version of perl is compiled without perlio, and this form of open is not supported.

(W pipe) You tried to say "open(CMD, "|cmd|")", which is not supported. You can try any of several modules in the Perl library to do this, such as IPC::Open2. Alternately, direct the pipe's output to a file using ">", and then read it in under a different file handle.
(F) An error peculiar to VMS. Perl does its own command line redirection, and couldn't open the file specified after '2>' or '2>>' on the command line for writing.
(F) An error peculiar to VMS. Perl does its own command line redirection, and couldn't open the file specified after '<' on the command line for reading.
(F) An error peculiar to VMS. Perl does its own command line redirection, and couldn't open the file specified after '>' or '>>' on the command line for writing.
(P) An error peculiar to VMS. Perl does its own command line redirection, and couldn't open the pipe into which to send data destined for stdout.
(F) The script you specified can't be opened for the indicated reason.

If you're debugging a script that uses #!, and normally relies on the shell's $PATH search, the -S option causes perl to do that search, so you don't have to type the path or `which $scriptname`.

(S) A warning peculiar to VMS. Perl tried to read an element of %ENV from the CRTL's internal environment array and discovered the array was missing. You need to figure out where your CRTL misplaced its environ or define PERL_ENV_TABLES (see perlvms) so that environ is not searched.
(F) A "my", "our" or "state" declaration was found within another declaration, such as "my ($x, my($y), $z)" or "our (my $x)".
(F) A "redo" statement was executed to restart the current block, but there isn't a current block. Note that an "if" or "else" block doesn't count as a "loopish" block, as doesn't a block given to sort(), map() or grep(). You can usually double the curlies to get the same effect though, because the inner curlies will be considered a block that loops once. See "redo" in perlfunc.
(S inplace) You requested an inplace edit without creating a backup file. Perl was unable to remove the original file to replace it with the modified file. The file was left unmodified.
(F) When closed implicitly, the temporary file for in-place editing couldn't be renamed to the original filename.
(F) The rename done by the -i switch failed for some reason, probably because you don't have write permission to the directory.
(P) An error peculiar to VMS. Perl thought stdin was a pipe, and tried to reopen it to accept binary data. Alas, it failed.
(F) There is a hard limit to how big a character code point can be due to the fundamental properties of UTF-8, especially on EBCDIC platforms. The given code point exceeds that. The only work-around is to not use such a large code point.
(F) You called reset('E') or similar, which tried to reset all variables in the current package beginning with "E". In the main package, that includes %ENV. Resetting %ENV is not supported on some systems, notably VMS.
(F)(P) Error resolving overloading specified by a method name (as opposed to a subroutine reference): no such method callable via the package. If the method name is "???", this is an internal error.
(F) Perl detected an attempt to return illegal lvalues (such as temporary or readonly values) from a subroutine used as an lvalue. This is not allowed.
(F) The return statement was executed in mainline code, that is, where there was no subroutine call to return out of. See perlsub.
(F) You tried to return a complete array or hash from an lvalue subroutine, but you called the subroutine in a way that made Perl think you meant to return only one value. You probably meant to write parentheses around the call to the subroutine, which tell Perl that the call should be in list context.
(F) For ordinary real numbers, you can't take the logarithm of a negative number or zero. There's a Math::Complex package that comes standard with Perl, though, if you really want to do that for the negative numbers.
(F) For ordinary real numbers, you can't take the square root of a negative number. There's a Math::Complex package that comes standard with Perl, though, if you really want to do that.
(F) You can't undefine a routine that's currently running. You can, however, redefine it while it's running, and you can even undef the redefined subroutine while the old routine is running. Go figure.
(F) You attempted to unweaken something that was not a reference. Only references can be unweakened.
(P) The internal sv_upgrade routine adds "members" to an SV, making it into a more specialized kind of SV. The top several SV types are so specialized, however, that they cannot be interconverted. This message indicates that such a conversion was attempted.
(F) You tried to call perl with the -m switch, but you put something other than "=" after the module name.
(F) You tried to use a hash as a reference, as in "%foo->{"bar"}" or "%$ref->{"hello"}". Versions of perl <= 5.22.0 used to allow this syntax, but shouldn't have. This was deprecated in perl 5.6.1.
(F) You tried to use an array as a reference, as in "@foo->[23]" or "@$ref->[99]". Versions of perl <= 5.22.0 used to allow this syntax, but shouldn't have. This was deprecated in perl 5.6.1.
(F) The internal routine that does method lookup was handed a symbol table that doesn't have a name. Symbol tables can become anonymous for example by undefining stashes: "undef %Some::Package::".
(F) A value used as either a hard reference or a symbolic reference must be a defined value. This helps to delurk some insidious errors.
(F) Only hard references are allowed by "strict refs". Symbolic references are disallowed. See perlref.
(F) The first time the "%!" hash is used, perl automatically loads the Errno.pm module. The Errno module is expected to tie the %! hash to provide symbolic names for $! errno values.
(F) A type cannot be forced to have both big-endian and little-endian byte-order at the same time, so this combination of modifiers is not allowed. See "pack" in perlfunc.
(F) defined() is not useful on arrays because it checks for an undefined scalar value. If you want to see if the array is empty, just use "if (@array) { # not empty }" for example.
(F) defined() is not usually right on hashes.

Although "defined %hash" is false on a plain not-yet-used hash, it becomes true in several non-obvious circumstances, including iterators, weak references, stash names, even remaining true after "undef %hash". These things make "defined %hash" fairly useless in practice, so it now generates a fatal error.

If a check for non-empty is what you wanted then just put it in boolean context (see "Scalar values" in perldata):

    if (%hash) {
       # not empty
    }

If you had "defined %Foo::Bar::QUUX" to check whether such a package variable exists then that's never really been reliable, and isn't a good way to enquire about the features of a package, or whether it's loaded, etc.

(P) The parser got confused when trying to parse a "foreach" loop.
(F) You tried to declare a magical variable as a lexical variable. This is not allowed, because the magic can be tied to only one location (namely the global variable) and it would be incredibly confusing to have variables in your program that looked like magical variables but weren't.
(F) You attempted to force a different byte-order on a type that is already inside a group with a byte-order modifier. For example you cannot force little-endianness on a type that is inside a big-endian group.
(F) The global variables $a and $b are reserved for sort comparisons. You mentioned $a or $b in the same line as the <=> or cmp operator, and the variable had earlier been declared as a lexical variable. Either qualify the sort variable with the package name, or rename the lexical variable.
(F) You've mixed up your reference types. You have to dereference a reference of the type needed. You can use the ref() function to test the type of the reference, if need be.
(F) You've told Perl to dereference a string, something which "use strict" blocks to prevent it happening accidentally. See "Symbolic references" in perlref. This can be triggered by an "@" or "$" in a double-quoted string immediately before interpolating a variable, for example in "user @$twitter_id", which says to treat the contents of $twitter_id as an array reference; use a "\" to have a literal "@" symbol followed by the contents of $twitter_id: "user \@$twitter_id".
(F) The compiler tried to interpret a bracketed expression as a subscript. But to the left of the brackets was an expression that didn't look like a hash or array reference, or anything else subscriptable.
(W syntax) In an ordinary expression, backslash is a unary operator that creates a reference to its argument. The use of backslash to indicate a backreference to a matched substring is valid only as part of a regular expression pattern. Trying to do this in ordinary Perl code produces a value that prints out looking like SCALAR(0xdecaf). Use the $1 form instead.
(F) You attempted to weaken something that was not a reference. Only references can be weakened.
(F) You have used a when() block that is neither inside a "foreach" loop nor a "given" block. (Note that this error is issued on exit from the "when" block, so you won't get the error if the match fails, or if you use an explicit "continue".)
(F) You tried to repeat a constant value (often the undefined value) with an assignment operator, which implies modifying the value itself. Perhaps you need to copy the value to a temporary, and repeat that.
(F) You tried to use the "try" and "catch" syntax of "use feature 'try'" but did not include the error variable in the "catch" block. The parenthesized variable name is not optional, unlike in some other forms of syntax you may be familiar with from CPAN modules or other languages.

The required syntax is

    try { ... }
    catch ($var) { ... }
(W deprecated) Once you have a "use VERSION" statement in scope, any "use VERSION" statement that requests a different version is now deprecated, due to the increasing complexity of swapping from one prevailing version to another.

It is suggested that you do not try to mix multiple different version declarations within the same file as it leads to complex behaviours about the visibility of features and builtin functions, as well as confusing human readers.

If it is essential to have different "use VERSION" declarations in different regions of the same file, you should surround each one by its own enclosing scope so the two do not mix.

  {
    use v5.20;
    ...
  }
  {
    use v5.36;
    ...
  }
(F) In "\cX", X must be a printable (non-control) ASCII character.

Note that ASCII characters that don't map to control characters are discouraged, and will generate the warning (when enabled) ""\c%c" is more clearly written simply as "%s"".

(F) (In the above the %c is replaced by either "p" or "P".) You specified something that isn't a legal Unicode property name. Most Unicode properties are specified by "\p{...}". But if the name is a single character one, the braces may be omitted.
(W pack) You said 

    pack("C", $x)

where $x is either less than 0 or more than 255; the "C" format is only for encoding native operating system characters (ASCII, EBCDIC, and so on) and not for Unicode characters, so Perl behaved as if you meant

    pack("C", $x & 255)

If you actually want to pack Unicode codepoints, use the "U" format instead.

(W pack) You said 

    pack("c", $x)

where $x is either less than -128 or more than 127; the "c" format is only for encoding native operating system characters (ASCII, EBCDIC, and so on) and not for Unicode characters, so Perl behaved as if you meant

    pack("c", $x & 255);

If you actually want to pack Unicode codepoints, use the "U" format instead.

(W unpack) You tried something like 

   unpack("H", "\x{2a1}")

where the format expects to process a byte (a character with a value below 256), but a higher value was provided instead. Perl uses the value modulus 256 instead, as if you had provided:

   unpack("H", "\x{a1}")
(W pack) You said 

    pack("U0W", $x)

where $x is either less than 0 or more than 255. However, "U0"-mode expects all values to fall in the interval [0, 255], so Perl behaved as if you meant:

    pack("U0W", $x & 255)
(W pack) You tried something like 

   pack("u", "\x{1f3}b")

where the format expects to process a sequence of bytes (character with a value below 256), but some of the characters had a higher value. Perl uses the character values modulus 256 instead, as if you had provided:

   pack("u", "\x{f3}b")
(W unpack) You tried something like 

   unpack("s", "\x{1f3}b")

where the format expects to process a sequence of bytes (character with a value below 256), but some of the characters had a higher value. Perl uses the character values modulus 256 instead, as if you had provided:

   unpack("s", "\x{f3}b")
(F) You defined a character name which had multiple space characters in a row. Change them to single spaces. Usually these names are defined in the ":alias" import argument to "use charnames", but they could be defined by a translator installed into $^H{charnames}. See "CUSTOM ALIASES" in charnames.
(W unopened) You tried chdir() on a filehandle that was never opened.
"\c%c" is more clearly written simply as "%s"
(W syntax) The "\cX" construct is intended to be a way to specify non-printable characters. You used it for a printable one, which is better written as simply itself, perhaps preceded by a backslash for non-word characters. Doing it the way you did is not portable between ASCII and EBCDIC platforms.
(F) You attempted to specify a second superclass for a "class" by using the ":isa" attribute, when one is already specified. Unlike classes whose instances are created with "bless", classes created via the "class" keyword cannot have more than one superclass.
(F) You specified an attribute for a class that would require a value to be passed in parentheses, but did not provide one. Remember that whitespace is not permitted between the attribute name and its value; you must write this as 

    class Example::Class :attr(VALUE) ...
(S experimental::class) This warning is emitted if you use the "class" keyword of "use feature 'class'". This keyword is currently experimental and its behaviour may change in future releases of Perl.
(F) When creating a subclass using the "class" ":isa" attribute, the named superclass must also be a real class created using the "class" keyword.
(F) Creating a new thread inside the "s///" operator is not supported.
(W io) The dirhandle you tried to close is either closed or not really a dirhandle. Check your control flow.
(W unopened) You tried to close a filehandle that was never opened.
(F) If a closure has attributes, the subroutine passed to an attribute handler is the prototype that is cloned when a new closure is created. This subroutine cannot be called.
(F) The \C character class used to allow a match of single byte within a multi-byte utf-8 character, but was removed in v5.24 as it broke encapsulation and its implementation was extremely buggy. If you really need to process the individual bytes, you probably want to convert your string to one where each underlying byte is stored as a character, with utf8::encode().
(F) You had a (sub-)template that ends with a '/'. There must be another template code following the slash. See "pack" in perlfunc.
(S non_unicode portable) You had a code point that has never been in any standard, so it is likely that languages other than Perl will NOT understand it. This code point also will not fit in a 32-bit word on ASCII platforms and therefore is non-portable between systems.

At one time, it was legal in some standards to have code points up to 0x7FFF_FFFF, but not higher, and this code point is higher.

Acceptance of these code points is a Perl extension, and you should expect that nothing other than Perl can handle them; Perl itself on EBCDIC platforms before v5.24 does not handle them.

Perl also makes no guarantees that the representation of these code points won't change at some point in the future, say when machines become available that have larger than a 64-bit word. At that time, files containing any of these, written by an older Perl might require conversion before being readable by a newer Perl.

(S non_unicode) You had a code point above the Unicode maximum of U+10FFFF.

Perl allows strings to contain a superset of Unicode code points, but these may not be accepted by other languages/systems. Further, even if these languages/systems accept these large code points, they may have chosen a different representation for them than the UTF-8-like one that Perl has, which would mean files are not exchangeable between them and Perl.

On EBCDIC platforms, code points above 0x3FFF_FFFF have a different representation in Perl v5.24 than before, so any file containing these that was written before that version will require conversion before being readable by a later Perl.

%s: Command not found
(A) You've accidentally run your script through csh or another shell instead of Perl. Check the #! line, or manually feed your script into Perl yourself. The #! line at the top of your file could look like 

  #!/usr/bin/perl
%s: command not found
(A) You've accidentally run your script through bash or another shell instead of Perl. Check the #! line, or manually feed your script into Perl yourself. The #! line at the top of your file could look like 

  #!/usr/bin/perl
%s: command not found: %s
(A) You've accidentally run your script through zsh or another shell instead of Perl. Check the #! line, or manually feed your script into Perl yourself. The #! line at the top of your file could look like 

  #!/usr/bin/perl
(F) Perl could not compile a file specified in a "require" statement. Perl uses this generic message when none of the errors that it encountered were severe enough to halt compilation immediately.
(W closed) You tried to do a connect on a closed socket. Did you forget to check the return value of your socket() call? See "connect" in perlfunc.
(F) The subroutine registered to handle constant overloading (see overload) or a custom charnames handler (see "CUSTOM TRANSLATORS" in charnames) returned an undefined value.
(F) The parser found inconsistencies while attempting to define an overloaded constant. Perhaps you forgot to load the corresponding overload pragma?
(F) A constant value (perhaps declared using the "use constant" pragma) is being dereferenced, but it amounts to the wrong type of reference. The message indicates the type of reference that was expected. This usually indicates a syntax error in dereferencing the constant value. See "Constant Functions" in perlsub and constant.
(F) You wrote something like 

    my $var;
    $sub = sub () { $var };

but $var is referenced elsewhere and could be modified after the "sub" expression is evaluated. Either it is explicitly modified elsewhere ("$var = 3") or it is passed to a subroutine or to an operator like "printf" or "map", which may or may not modify the variable.

Traditionally, Perl has captured the value of the variable at that point and turned the subroutine into a constant eligible for inlining. In those cases where the variable can be modified elsewhere, this breaks the behavior of closures, in which the subroutine captures the variable itself, rather than its value, so future changes to the variable are reflected in the subroutine's return value.

This usage was deprecated, and as of Perl 5.32 is no longer allowed, making it possible to change the behavior in the future.

If you intended for the subroutine to be eligible for inlining, then make sure the variable is not referenced elsewhere, possibly by copying it:

    my $var2 = $var;
    $sub = sub () { $var2 };

If you do want this subroutine to be a closure that reflects future changes to the variable that it closes over, add an explicit "return":

    my $var;
    $sub = sub () { return $var };
(W redefine)(S) You redefined a subroutine which had previously been eligible for inlining. See "Constant Functions" in perlsub for commentary and workarounds.
(W misc) You undefined a subroutine which had previously been eligible for inlining. See "Constant Functions" in perlsub for commentary and workarounds.
(F) The parser found inconsistencies either while attempting to define an overloaded constant, or when trying to find the character name specified in the "\N{...}" escape. Perhaps you forgot to load the corresponding overload pragma?
:const is not permitted on named subroutines
(F) The "const" attribute causes an anonymous subroutine to be run and its value captured at the time that it is cloned. Named subroutines are not cloned like this, so the attribute does not make sense on them.
(F) The method which overloads "=" is buggy. See "Copy Constructor" in overload.
&CORE::%s cannot be called directly
(F) You tried to call a subroutine in the "CORE::" namespace with &foo syntax or through a reference. Some subroutines in this package cannot yet be called that way, but must be called as barewords. Something like this will work: 

    BEGIN { *shove = \&CORE::push; }
    shove @array, 1,2,3; # pushes on to @array
(F) The CORE:: namespace is reserved for Perl keywords.
(P) This is either an error in Perl, or, if you're using one, your custom regular expression engine. If not the latter, report the problem to <https://github.com/Perl/perl5/issues/new/choose>.
(P) The regular expression engine got confused by what the regular expression compiler gave it.
(P) The regular expression engine got passed a regexp program without a valid magic number.
(P) The malloc package that comes with Perl had an internal failure.
(F) You had an unpack template indicating a counted-length string, but you have also specified an explicit size for the string. See "pack" in perlfunc.
(S experimental::declared_refs) This warning is emitted if you use a reference constructor on the right-hand side of "my", "state", "our", or "local". Simply suppress the warning if you want to use the feature, but know that in doing so you are taking the risk of using an experimental feature which may change or be removed in a future Perl version: 

    no warnings "experimental::declared_refs";
    use feature "declared_refs";
    $fooref = my \$foo;
(W recursion) This subroutine has called itself (directly or indirectly) 100 times more than it has returned. This probably indicates an infinite recursion, unless you're writing strange benchmark programs, in which case it indicates something else.

This threshold can be changed from 100, by recompiling the perl binary, setting the C pre-processor macro "PERL_SUB_DEPTH_WARN" to the desired value.

(?(DEFINE)....) does not allow branches in regex; marked by <-- HERE in m/%s/
(F) You used something like "(?(DEFINE)...|..)" which is illegal. The most likely cause of this error is that you left out a parenthesis inside of the "...." part.

The <-- HERE shows whereabouts in the regular expression the problem was discovered.

%s defines neither package nor VERSION--version check failed
(F) You said something like "use Module 42" but in the Module file there are neither package declarations nor a $VERSION.
(F) The argument to "delete" must be either a hash or array element, such as: 

    $foo{$bar}
    $ref->{"susie"}[12]

or a hash or array slice, such as:

    @foo[$bar, $baz, $xyzzy]
    $ref->[12]->@{"susie", "queue"}

or a hash key/value or array index/value slice, such as:

    %foo[$bar, $baz, $xyzzy]
    $ref->[12]->%{"susie", "queue"}
(F) In a here document construct like "<<FOO", the label "FOO" is too long for Perl to handle. You have to be seriously twisted to write code that triggers this error.
(F) A DESTROY() method created a new reference to the object which is just being DESTROYed. Perl is confused, and prefers to abort rather than to create a dangling reference.
See "500 Server error".
%s did not return a true value
(F) A required (or used) file must return a true value to indicate that it compiled correctly and ran its initialization code correctly. It's traditional to end such a file with a "1;", though any true value would do. See "require" in perlfunc.
(Did you mean &%s instead?)
(W misc) You probably referred to an imported subroutine &FOO as $FOO or some such.
(Did you mean "local" instead of "our"?)
(W shadow) Remember that "our" does not localize the declared global variable. You have declared it again in the same lexical scope, which seems superfluous.
(Did you mean $ or @ instead of %?)
(W) You probably said %hash{$key} when you meant $hash{$key} or @hash{@keys}. On the other hand, maybe you just meant %hash and got carried away.
(F) You passed die() an empty string (the equivalent of "die """) or you called it with no args and $@ was empty.
See "500 Server error".
%s does not define %s::VERSION--version check failed
(F) You said something like "use Module 42" but the Module did not define a $VERSION.
'/' does not take a repeat count in %s
(F) You cannot put a repeat count of any kind right after the '/' code. See "pack" in perlfunc.
(D deprecated::dot_in_inc) Previously " do "somefile"; " would search the current directory for the specified file. Since perl v5.26.0, . has been removed from @INC by default, so this is no longer true. To search the current directory (and only the current directory) you can write " do "./somefile"; ".
'%s' does not appear to be an imported builtin function
(F) An attempt was made to remove a previously-imported lexical from builtin by using the "unimport" method (likely via "no builtin ..." syntax), but the requested function has not been imported into the current scope.
(P) "PerlIO_getname", a perl internal I/O function specific to VMS, was somehow called on another platform. This should not happen.
(P) The internal handling of magical variables has been cursed.
(F) A "use VERSION" statement that requests a version below v5.11 (when the effects of "use strict" would be disabled) has been found after a previous declaration of one having a larger number (which would have enabled these effects). Because of a change to the way that "use VERSION" interacts with the strictness flags, this is no longer supported.
(Do you need to predeclare %s?)
(S syntax) This is an educated guess made in conjunction with the message "%s found where operator expected". It often means a subroutine or module name is being referenced that hasn't been declared yet. This may be because of ordering problems in your file, or because of a missing "sub", "package", "require", or "use" statement. If you're referencing something that isn't defined yet, you don't actually have to define the subroutine or package before the current location. You can use an empty "sub foo;" or "package FOO;" to enter a "forward" declaration.
(F) You used the obsolete dump() built-in function. That was deprecated in Perl 5.8.0. As of Perl 5.30 it must be written in fully qualified format: CORE::dump().

See "dump" in perlfunc.

(F) Your machine doesn't support dump/undump.
(S malloc) An internal routine called free() on something that had already been freed.
(W unpack) You have applied the same modifier more than once after a type in a pack template. See "pack" in perlfunc.
(W syntax) You called each on an anonymous hash or array. Since a new hash or array is created each time, each() will restart iterating over your hash or array every time.
(S syntax) There is no keyword "elseif" in Perl because Larry thinks it's ugly. Your code will be interpreted as an attempt to call a method named "elseif" for the class returned by the following block. This is unlikely to be what you want.
(F) You used something like "\b{}", "\B{}", "\o{}", "\p", "\P", or "\x" without specifying anything for it to operate on.

Unfortunately, for backwards compatibility reasons, an empty "\x" is legal outside "use re 'strict'" and expands to a NUL character.

(W regexp) (only under "use re 'strict'") "(?)" does nothing, so perhaps this is a typo.
${^ENCODING} is no longer supported
(F) The special variable "${^ENCODING}", formerly used to implement the "encoding" pragma, is no longer supported as of Perl 5.26.0.

Setting it to anything other than "undef" is a fatal error as of Perl 5.28.

${^HOOK}{%s} may only be a CODE reference or undef
(F) You attempted to assign something other than undef or a CODE ref to "%{^HOOK}". Hooks may only be CODE refs. See "%{^HOOK}" in perlvar for details.
(F) You attempted to assign something other than undef or a CODE ref to "%{^HOOK}". Hooks may only be CODE refs. See "%{^HOOK}" in perlvar for details.
(F) While under the "use filetest" pragma, switching the real and effective uids or gids failed.
%ENV is aliased to %s
(F) You're running under taint mode, and the %ENV variable has been aliased to another hash, so it doesn't reflect anymore the state of the program's environment. This is potentially insecure.
(F) An error peculiar to VMS. Because Perl may have to deal with file specifications in either VMS or Unix syntax, it converts them to a single form when it must operate on them directly. Either you've passed an invalid file specification to Perl, or you've found a case the conversion routines don't handle. Drat.
(F) An error was encountered in handling a user-defined property ("User-Defined Character Properties" in perlunicode). These are programmer written subroutines, hence subject to errors that may prevent them from compiling or running. The calls to these subs are "eval"'d, and if there is a failure, this message is raised, using the contents of $@ from the failed "eval".

Another possibility is that tainted data was encountered somewhere in the chain of expanding the property. If so, the message wording will indicate that this is the problem. See "Insecure user-defined property %s".

(F) Perl detected tainted data when trying to compile a regular expression that contains the "(?{ ... })" zero-width assertion, which is unsafe. See "(?{ code })" in perlre, and perlsec.
(F) Perl tried to compile a regular expression containing the "(?{ ... })" zero-width assertion at run time, as it would when the pattern contains interpolated values. Since that is a security risk, it is not allowed. If you insist, you may still do this by using the "re 'eval'" pragma or by explicitly building the pattern from an interpolated string at run time and using that in an eval(). See "(?{ code })" in perlre.
(F) A regular expression contained the "(?{ ... })" zero-width assertion, but that construct is only allowed when the "use re 'eval'" pragma is in effect. See "(?{ code })" in perlre.
(F) You used a pattern that nested too many EVAL calls without consuming any text. Restructure the pattern so that text is consumed.

The <-- HERE shows whereabouts in the regular expression the problem was discovered.

(F) The contents of a <> operator may not exceed the maximum size of a Perl identifier. If you're just trying to glob a long list of filenames, try using the glob() operator, or put the filenames into a variable and glob that.
(F) The "exec" function is not implemented on some systems, e.g. Catamount. See perlport.
%sExecution of %s aborted due to compilation errors.
(F) The final summary message when a Perl compilation fails.
(F) The final summary message when a Perl compilation fails.
(F) The argument to "exists" must be a hash or array element or a subroutine with an ampersand, such as: 

    $foo{$bar}
    $ref->{"susie"}[12]
    &do_something
(F) The argument to "exists" for "exists &sub" must be a subroutine name, and not a subroutine call. "exists &sub()" will generate this error.
(W exiting) You are exiting an eval by unconventional means, such as a goto, or a loop control statement.
(W exiting) You are exiting a format by unconventional means, such as a goto, or a loop control statement.
(W exiting) You are exiting a rather special block construct (like a sort block or subroutine) by unconventional means, such as a goto, or a loop control statement. See "sort" in perlfunc.
(W exiting) You are exiting a subroutine by unconventional means, such as a goto, or a loop control statement.
(W exiting) You are exiting a substitution by unconventional means, such as a return, a goto, or a loop control statement.
(F) The type of a reference given to "export_lexically" in builtin did not match the sigil of the preceding name, or the value was not a reference at all.
(F) You wrote something like 

 (?13

to denote a capturing group of the form "(?PARNO)", but omitted the ")".

(F) It looked like you were attempting to interpolate an already-compiled extended character class, like so: 

 my $thai_or_lao = qr/(?[ \p{Thai} + \p{Lao} ])/;
 ...
 qr/(?[ \p{Digit} & $thai_or_lao ])/;

But the marked code isn't syntactically correct to be such an interpolated class.

(F) To do aliasing via references, you must first enable the feature: 

    no warnings "experimental::refaliasing";
    use feature "refaliasing";
    \$x = \$y;
(F) An experimental feature added in Perl 5.14 allowed "each", "keys", "push", "pop", "shift", "splice", "unshift", and "values" to be called with a scalar argument. This experiment is considered unsuccessful, and has been removed. The "postderef" feature may meet your needs better.
(F) To use subroutine signatures, you must first enable them: 

    use feature "signatures";
    sub foo ($left, $right) { ... }
(W misc) You are blessing a reference to a zero length string. This has the effect of blessing the reference into the package main. This is usually not what you want. Consider providing a default target package, e.g. bless($ref, $p || 'MyPackage');
(F) "export_lexically" in builtin was called at runtime. Because it creates new names in the lexical scope currently being compiled, it can only be called from code inside "BEGIN" block in that scope.
%s: Expression syntax
(A) You've accidentally run your script through csh instead of Perl. Check the #! line, or manually feed your script into Perl yourself.
%s failed--call queue aborted
(F) An untrapped exception was raised while executing a UNITCHECK, CHECK, INIT, or END subroutine. Processing of the remainder of the queue of such routines has been prematurely ended.
(F) Closing an output file from in-place editing, as with the "-i" command-line switch, failed.
(W regexp)(F) A character class range must start and end at a literal character, not another character class like "\d" or "[:alpha:]". The "-" in your false range is interpreted as a literal "-". In a "(?[...])" construct, this is an error, rather than a warning. Consider quoting the "-", "\-". The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.
(P) An error peculiar to VMS. Something untoward happened in a VMS system service or RTL routine; Perl's exit status should provide more details. The filename in "at %s" and the line number in "line %d" tell you which section of the Perl source code is distressed.
(F) Your machine apparently doesn't implement fcntl(). What is this, a PDP-11 or something?
(F) A tied array claimed to have a negative number of elements, which is not possible.
(F) A field may have at most one application of the ":param" attribute to assign a parameter name to it; once applied a second one is not allowed.
(F) You specified an attribute for a field that would require a value to be passed in parentheses, but did not provide one. Remember that whitespace is not permitted between the attribute name and its value; you must write this as 

    field $var :attr(VALUE) ...
(S experimental::class) This warning is emitted if you use the "field" keyword of "use feature 'class'". This keyword is currently experimental and its behaviour may change in future releases of Perl.
(F) An attempt was made to access a field variable of a class from code that does not appear inside the body of a "method" subroutine. This is not permitted, as only methods will have access to the fields of an instance.
(F) An attempt was made to access a field variable of a class, from a method of another class nested inside the one that actually defined it. This is not permitted, as only methods defined by a given class are permitted to access fields of that class.
(W pack) Each line in an uuencoded string starts with a length indicator which can't encode values above 63. So there is no point in asking for a line length bigger than that. Perl will behave as if you specified "u63" as the format.
(W io) You tried to write on a read-only filehandle. If you intended it to be a read-write filehandle, you needed to open it with "+<" or "+>" or "+>>" instead of with "<" or nothing. If you intended only to write the file, use ">" or ">>". See "open" in perlfunc.
(W io) You tried to read from a filehandle opened only for writing, If you intended it to be a read/write filehandle, you needed to open it with "+<" or "+>" or "+>>" instead of with ">". If you intended only to read from the file, use "<". See "open" in perlfunc. Another possibility is that you attempted to open filedescriptor 0 (also known as STDIN) for output (maybe you closed STDIN earlier?).
(W io) You opened for reading a filehandle that got the same filehandle id as STDOUT or STDERR. This occurred because you closed STDOUT or STDERR previously.
(W io) You opened for writing a filehandle that got the same filehandle id as STDIN. This occurred because you closed STDIN previously.
(W io) You opened for reading a filehandle that got the same filehandle id as STDOUT or STDERR. This occurred because you closed the handle previously.
(F) You must now decide whether the final $ in a string was meant to be a literal dollar sign, or was meant to introduce a variable name that happens to be missing. So you have to put either the backslash or the name.
(S experimental::defer) The "defer" block modifier is experimental. If you want to use the feature, disable the warning with "no warnings 'experimental::defer'", but know that in doing so you are taking the risk that your code may break in a future Perl version.
(W closed) The filehandle you're attempting to flock() got itself closed some time before now. Check your control flow. flock() operates on filehandles. Are you attempting to call flock() on a dirhandle by the same name?
(S experimental::for_list) This warning is emitted if you use "for" to iterate multiple values at a time. This syntax is currently experimental and its behaviour may change in future releases of Perl.
(S inplace) You had "|-" or "-|" in @ARGV and tried to use "<>" to read from it.

Previously this would fork and produce a confusing error message.

(F) A format must be terminated by a line with a solitary dot. Perl got to the end of your file without finding such a line.
(W redefine) You redefined a format. To suppress this warning, say 

    {
        no warnings 'redefine';
        eval "format NAME =...";
    }
(W syntax) You said 

    if ($foo = 123)

when you meant

    if ($foo == 123)

(or something like that).

%s found where operator expected
(S syntax) The Perl lexer knows whether to expect a term or an operator. If it sees what it knows to be a term when it was expecting to see an operator, it gives you this warning. Usually it indicates that an operator or delimiter was omitted, such as a semicolon.
(S) A warning from the GDBM_File extension that a store failed.
(F) Your C library apparently doesn't implement gethostent(), probably because if it did, it'd feel morally obligated to return every hostname on the Internet.
(W closed) You tried to get a socket or peer socket name on a closed socket. Did you forget to check the return value of your socket() call?
(S) A warning peculiar to VMS. The call to "sys$getuai" underlying the "getpwnam" operator returned an invalid UIC.
(W closed) You tried to get a socket option on a closed socket. Did you forget to check the return value of your socket() call? See "getsockopt" in perlfunc.
(F) You called PerlIO::get_layers() with an unknown argument. Legal arguments are provided in key/value pairs, with the keys being one of "input", "output" or "detail", followed by a boolean.
(D deprecated::smartmatch) "given" depends on smartmatch, which is deprecated. It will be removed in Perl 5.42. See the explanation under "Experimental Details on given and when" in perlsyn.
(F) You've said "use strict" or "use strict vars", which indicates that all variables must either be lexically scoped (using "my" or "state"), declared beforehand using "our", or explicitly qualified to say which package the global variable is in (using "::").
(S glob) Something went wrong with the external program(s) used for "glob" and "<*.c>". Usually, this means that you supplied a "glob" pattern that caused the external program to fail and exit with a nonzero status. If the message indicates that the abnormal exit resulted in a coredump, this may also mean that your csh (C shell) is broken. If so, you should change all of the csh-related variables in config.sh: If you have tcsh, make the variables refer to it as if it were csh (e.g. "full_csh='/usr/bin/tcsh'"); otherwise, make them all empty (except that "d_csh" should be 'undef') so that Perl will think csh is missing. In either case, after editing config.sh, run "./Configure -S" and rebuild Perl.
(F) The lexer saw a left angle bracket in a place where it was expecting a term, so it's looking for the corresponding right angle bracket, and not finding it. Chances are you left some needed parentheses out earlier in the line, and you really meant a "less than".
(W overflow) You called "gmtime" with a number that it could not handle: too large, too small, or NaN. The returned value is "undef".
(W overflow) You called "gmtime" with a number that was larger than it can reliably handle and "gmtime" probably returned the wrong date. This warning is also triggered with NaN (the special not-a-number value).
(W overflow) You called "gmtime" with a number that was smaller than it can reliably handle and "gmtime" probably returned the wrong date.
(P) An error peculiar to OS/2. Most probably you're using an obsolete version of Perl, and this should not happen anyway.
(F) Unlike with "next" or "last", you're not allowed to goto an unspecified destination. See "goto" in perlfunc.
(F) You tried to call a subroutine with "goto &sub" syntax, but the indicated subroutine hasn't been defined, or if it was, it has since been undefined.
(F) Group names must follow the rules for perl identifiers, meaning they must start with a non-digit word character. A common cause of this error is using (?&0) instead of (?0). See perlre.
()-group starts with a count
(F) A ()-group started with a count. A count is supposed to follow something: a template character or a ()-group. See "pack" in perlfunc.
%s had compilation errors.
(F) The final summary message when a "perl -c" fails.
(S internal) A routine asked for a symbol from a symbol table that ought to have existed already, but for some reason it didn't, and had to be created on an emergency basis to prevent a core dump.
%s has too many errors
(F) The parser has given up trying to parse the program after 10 errors. Further error messages would likely be uninformative.
(W overflow) The hexadecimal floating point has a larger exponent than the floating point supports.
(W overflow) The hexadecimal floating point has a smaller exponent than the floating point supports. With the IEEE 754 floating point, this may also mean that the subnormals (formerly known as denormals) are being used, which may or may not be an error.
(F) Something went horribly bad in hexadecimal float handling.
(W overflow) The hexadecimal floating point literal had more bits in the mantissa (the part between the 0x and the exponent, also known as the fraction or the significand) than the floating point supports.
(W overflow) The hexadecimal floating point had internally more digits than could be output. This can be caused by unsupported long double formats, or by 64-bit integers not being available (needed to retrieve the digits under some configurations).
(F) You have configured Perl to use long doubles but the internals of the long double format are unknown; therefore the hexadecimal float output is impossible.
(W portable) The hexadecimal number you specified is larger than 2**32-1 (4294967295) and therefore non-portable between systems. See perlport for more on portability concerns.
(F) Perl limits identifiers (names for variables, functions, etc.) to about 250 characters for simple names, and somewhat more for compound names (like $A::B). You've exceeded Perl's limits. Future versions of Perl are likely to eliminate these arbitrary limitations.
(W regexp) Named Unicode character escapes ("\N{...}") may return a zero-length sequence. When such an escape is used in a character class its behavior is not well defined. Check that the correct escape has been used, and the correct charname handler is in scope.
(W digit) Here %s is one of "binary", "octal", or "hex". You may have tried to use a digit other than one that is legal for the given type, such as only 0 and 1 for binary. For octals, this is raised only if the illegal character is an '8' or '9'. For hex, 'A' - 'F' and 'a' - 'f' are legal. Interpretation of the number stopped just before the offending digit or character.
(F) You used a digit other than 0 or 1 in a binary number.
(W illegalproto) An illegal character was found in a prototype declaration. The '_' in a prototype must be followed by a ';', indicating the rest of the parameters are optional, or one of '@' or '%', since those two will accept 0 or more final parameters.
(F) Perl normally treats carriage returns in the program text as it would any other whitespace, which means you should never see this error when Perl was built using standard options. For some reason, your version of Perl appears to have been built without this support. Talk to your Perl administrator.
(F) A parameter in a subroutine signature contained an unexpected character following the "$", "@" or "%" sigil character. Normally the sigil should be followed by the variable name or "=" etc. Perhaps you are trying use a prototype while in the scope of "use feature 'signatures'"? For example: 

    sub foo ($$) {}            # legal - a prototype
    use feature 'signatures;
    sub foo ($$) {}            # illegal - was expecting a signature
    sub foo ($x, $y)
            :prototype($$) {}  # legal
(W illegalproto) An illegal character was found in a prototype declaration. Legal characters in prototypes are $, @, %, *, ;, [, ], &, \, and +. Perhaps you were trying to write a subroutine signature but didn't enable that feature first ("use feature 'signatures'"), so your signature was instead interpreted as a bad prototype.
(F) When using the "sub" keyword to construct an anonymous subroutine, you must always specify a block of code. See perlsub.
(F) A subroutine was not declared correctly. See perlsub.
(F) You tried to divide a number by 0. Either something was wrong in your logic, or you need to put a conditional in to guard against meaningless input.
(F) You tried to divide a number by 0 to get the remainder. Most numbers don't take to this kindly.
(F) The number of bits in vec() (the third argument) must be a power of two from 1 to 32 (or 64, if your platform supports that).
(F) You used an 8 or 9 in an octal number.
(F) A parameter in a subroutine signature, was followed by something other than "=" introducing a default, "," or ")". 

    use feature 'signatures';
    sub foo ($=1) {}           # legal
    sub foo ($x = 1) {}        # legal
    sub foo ($x += 1) {}       # illegal
    sub foo ($x == 1) {}       # illegal
(F) You wrote something like 

 (?+foo)

The "+" is valid only when followed by digits, indicating a capturing group. See "(?PARNO)".

(F) The script run under suidperl was somehow illegal.
(X) The PERL5OPT environment variable may only be used to set the following switches: -[CDIMUdmtw].
(F) You specified a Unicode-like property name in a regular expression pattern (using "\p{}" or "\P{}") that Perl knows isn't an official Unicode property, and was likely meant to be a user-defined property name, but it can't be one of those, as they must begin with either "In" or "Is". Check the spelling. See also "Can't find Unicode property definition "%s"".
(W internal) A warning peculiar to VMS. Perl tried to read the CRTL's internal environ array, and encountered an element without the "=" delimiter used to separate keys from values. The element is ignored.
(W internal) A warning peculiar to VMS. Perl tried to read a logical name or CLI symbol definition when preparing to iterate over %ENV, and didn't see the expected delimiter between key and value, so the line was ignored.
(in cleanup) %s
(W misc) This prefix usually indicates that a DESTROY() method raised the indicated exception. Since destructors are usually called by the system at arbitrary points during execution, and often a vast number of times, the warning is issued only once for any number of failures that would otherwise result in the same message being repeated.

Failure of user callbacks dispatched using the "G_KEEPERR" flag could also result in this warning. See "G_KEEPERR" in perlcall.

(S experimental::args_array_with_signatures) An expression that implicitly involves the @_ arguments array was found in a subroutine that uses a signature. This is experimental because the interaction between the arguments array and parameter handling via signatures is not guaranteed to remain stable in any future version of Perl, and such code should be avoided.
(F) There was a syntax error within the "(?[ ])". This can happen if the expression inside the construct was completely empty, or if there are too many or few operands for the number of operators. Perl is not smart enough to give you a more precise indication as to what is wrong.
(F) The method resolution order (MRO) of the given class is not C3-consistent, and you have enabled the C3 MRO for this class. See the C3 documentation in mro for more information.
(F) You have an indented here-document where one or more of its lines have whitespace at the beginning that does not match the closing delimiter.

For example, line 2 below is wrong because it does not have at least 2 spaces, but lines 1 and 3 are fine because they have at least 2:

    if ($something) {
      print <<~EOF;
        Line 1
       Line 2 not
          Line 3
        EOF
    }

Note that tabs and spaces are compared strictly, meaning 1 tab will not match 8 spaces.

(F) You used a pattern that references itself without consuming any input text. You should check the pattern to ensure that recursive patterns either consume text or fail.
(F) A user-defined property ("User-Defined Character Properties" in perlunicode) can depend on the definitions of other user-defined properties. If the chain of dependencies leads back to this property, infinite recursion would occur, were it not for the check that raised this error.

Restructure your property definitions to avoid this.

(F) You tried to use the empty pattern inside of a regex code block, for instance "/(?{ s!!! })/", which resulted in re-executing the same pattern, which is an infinite loop which is broken by throwing an exception.
(F) "state" only permits initializing a single variable, specified without parentheses. So "state $x = 42" and "state @x = qw(a b c)" are allowed, but not "state ($x) = 42" or "(state $x) = 42". To initialize more than one "state" variable, initialize them one at a time.
%%s[%s] in scalar context better written as $%s[%s]
(W syntax) In scalar context, you've used an array index/value slice (indicated by %) to select a single element of an array. Generally it's better to ask for a scalar value (indicated by $). The difference is that $foo[&bar] always behaves like a scalar, both in the value it returns and when evaluating its argument, while %foo[&bar] provides a list context to its subscript, which can do weird things if you're expecting only one subscript. When called in list context, it also returns the index (what &bar returns) in addition to the value.
%%s{%s} in scalar context better written as $%s{%s}
(W syntax) In scalar context, you've used a hash key/value slice (indicated by %) to select a single element of a hash. Generally it's better to ask for a scalar value (indicated by $). The difference is that $foo{&bar} always behaves like a scalar, both in the value it returns and when evaluating its argument, while @foo{&bar} and provides a list context to its subscript, which can do weird things if you're expecting only one subscript. When called in list context, it also returns the key in addition to the value.
(F) You tried to do something that the tainting mechanism didn't like. The tainting mechanism is turned on when you're running setuid or setgid, or when you specify -T to turn it on explicitly. The tainting mechanism labels all data that's derived directly or indirectly from the user, who is considered to be unworthy of your trust. If any such data is used in a "dangerous" operation, you get this error. See perlsec for more information.
(F) You can't use system(), exec(), or a piped open in a setuid or setgid script if $ENV{PATH} contains a directory that is writable by the world. Also, the PATH must not contain any relative directory. See perlsec.
(F) You can't use system(), exec(), or a piped open in a setuid or setgid script if any of $ENV{PATH}, $ENV{IFS}, $ENV{CDPATH}, $ENV{ENV}, $ENV{BASH_ENV} or $ENV{TERM} are derived from data supplied (or potentially supplied) by the user. The script must set the path to a known value, using trustworthy data. See perlsec.
(F) Perl detected tainted data when trying to compile a regular expression that contains a call to a user-defined character property function, i.e. "\p{IsFoo}" or "\p{InFoo}". See "User-Defined Character Properties" in perlunicode and perlsec.
(F) The indexes and widths specified in the format string of printf() or sprintf() are too large. The numbers must not overflow the size of integers for your architecture.
(S overflow) The hexadecimal, octal or binary number you have specified either as a literal or as an argument to hex() or oct() is too big for your architecture, and has been converted to a floating point number. On a 32-bit architecture the largest hexadecimal, octal or binary number representable without overflow is 0xFFFFFFFF, 037777777777, or 0b11111111111111111111111111111111 respectively. Note that Perl transparently promotes all numbers to a floating point representation internally--subject to loss of precision errors in subsequent operations.
(S overflow) The number you have passed to srand is too big to fit in your architecture's integer representation. The number has been replaced with the largest integer supported (0xFFFFFFFF on 32-bit architectures). This means you may be getting less randomness than you expect, because different random seeds above the maximum will return the same sequence of random numbers.
(W overflow) Some portion of a version initialization is too large for the size of integers for your architecture. This is not a warning because there is no rational reason for a version to try and use an element larger than typically 2**32. This is usually caused by trying to use some odd mathematical operation as a version, like 100/9.
(P) Something went badly wrong in the regular expression parser. The <-- HERE shows whereabouts in the regular expression the problem was discovered.
(S) A warning peculiar to VMS. Perl keeps track of the number of times you've called "fork" and "exec", to determine whether the current call to "exec" should affect the current script or a subprocess (see "exec LIST" in perlvms). Somehow, this count has become scrambled, so Perl is making a guess and treating this "exec" as a request to terminate the Perl script and execute the specified command.
(S internal) Perl's internal routine that handles "printf" and "sprintf" formatting follows a slightly different set of rules when called from C or XS code. Specifically, formats consisting of digits followed by "p" (e.g., "%7p") are reserved for future use. If you see this message, then an XS module tried to call that routine with one such reserved format.
(P) Something went badly awry in the regular expression parser. The <-- HERE shows whereabouts in the regular expression the problem was discovered.
%s (...) interpreted as function
(W syntax) You've run afoul of the rule that says that any list operator followed by parentheses turns into a function, with all the list operators arguments found inside the parentheses. See "Terms and List Operators (Leftward)" in perlop.
(F) The two-character sequence "(?" in this context in a regular expression pattern should be an indivisible token, with nothing intervening between the "(" and the "?", but you separated them with whitespace.
(F) The two-character sequence "(*" in this context in a regular expression pattern should be an indivisible token, with nothing intervening between the "(" and the "*", but you separated them. Fix the pattern and retry.
(F) The indicated attribute for a subroutine or variable was not recognized by Perl or by a user-supplied handler. See attributes.
(F) The indicated attributes for a subroutine or variable were not recognized by Perl or by a user-supplied handler. See attributes.
(F) You tried to create a custom alias for a character name, with the ":alias" option to "use charnames" and the specified character in the indicated name isn't valid. See "CUSTOM ALIASES" in charnames.
(W syscalls) Embedded \0 characters in pathnames or other system call arguments produce a warning as of 5.20. The parts after the \0 were formerly ignored by system calls.
(F) Only certain characters are valid for character names. The indicated one isn't. See "CUSTOM ALIASES" in charnames.
(W printf) Perl does not understand the given format conversion. See "sprintf" in perlfunc.
(W regexp)(F) The numeric escape (for example "\xHH") of value < 256 didn't correspond to a single character through the conversion from the encoding specified by the encoding pragma. The escape was replaced with REPLACEMENT CHARACTER (U+FFFD) instead, except within "(?[   ])", where it is a fatal error. The <-- HERE shows whereabouts in the regular expression the escape was discovered.
(F) The character constant represented by "..." is not a valid hexadecimal number. Either it is empty, or you tried to use a character other than 0 - 9 or A - F, a - f in a hexadecimal number.
(F) The module argument to perl's -m and -M command-line options cannot contain single colons in the module name, but only in the arguments after "=". In other words, -MFoo::Bar=:baz is ok, but -MFoo:Bar=baz is not.
(F) You tried to "mro::set_mro("classname", "foo")" or "use mro 'foo'", where "foo" is not a valid method resolution order (MRO). Currently, the only valid ones supported are "dfs" and "c3", unless you have loaded a module that is a MRO plugin. See mro and perlmroapi.
(W utf8) You passed a negative number to "chr". Negative numbers are not valid character numbers, so it returns the Unicode replacement character (U+FFFD).
(F) You supplied a number to the -C option that either has extra leading zeroes or overflows perl's unsigned integer representation.
(S debugging) Perl was called with invalid debugger flags. Call perl with the -D option with no flags to see the list of acceptable values. See also "-Dletters" in perlrun.
(F) The pattern looks like a {min,max} quantifier, but the min or max could not be parsed as a valid number - either it has leading zeroes, or it represents too big a number to cope with. The <-- HERE shows where in the regular expression the problem was discovered. See perlre.
(F) The range specified in a character class had a minimum character greater than the maximum character. One possibility is that you forgot the "{}" from your ending "\x{}" - "\x" without the curly braces can go only up to "ff". The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.
(F) The range specified in the tr/// or y/// operator had a minimum character greater than the maximum character. See perlop.
(F) The capture group you specified can't possibly exist because the number you used is not within the legal range of possible values for this machine.
(F) Something other than a colon or whitespace was seen between the elements of an attribute list. If the previous attribute had a parenthesised parameter list, perhaps that list was terminated too soon. See attributes.
(W layer) When pushing layers onto the Perl I/O system, something other than a colon or whitespace was seen between the elements of a layer list. If the previous attribute had a parenthesised parameter list, perhaps that list was terminated too soon.
(F) A version number did not meet the "strict" criteria for versions. A "strict" version number is a positive decimal number (integer or decimal-fraction) without exponentiation or else a dotted-decimal v-string with a leading 'v' character and at least three components. The parenthesized text indicates which criteria were not met. See the version module for more details on allowed version formats.
(F) The given character is not a valid pack or unpack type. See "pack" in perlfunc.

(W) The given character is not a valid pack or unpack type but used to be silently ignored.

(F) A version number that is used to specify an import bundle during a "use builtin ..." statement must be formatted as ":MAJOR.MINOR" with an optional third component, which is ignored. Each component must be a number of 1 to 3 digits. No other characters are permitted. The value that was specified does not conform to these rules.
(F) A version number did not meet the "lax" criteria for versions. A "lax" version number is a positive decimal number (integer or decimal-fraction) without exponentiation or else a dotted-decimal v-string. If the v-string has fewer than three components, it must have a leading 'v' character. Otherwise, the leading 'v' is optional. Both decimal and dotted-decimal versions may have a trailing "alpha" component separated by an underscore character after a fractional or dotted-decimal component. The parenthesized text indicates which criteria were not met. See the version module for more details on allowed version formats.
(F) The internal structure of the version object was invalid. Perhaps the internals were modified directly in some way or an arbitrary reference was blessed into the "version" class.
(F) You wrote something like 

 qr/\P{name=KATAKANA LETTER AINU P}/
 qr/[^\p{name=KATAKANA LETTER AINU P}]/

This name actually evaluates to a sequence of two Katakana characters, not just a single one, and it is illegal to try to take the complement of a sequence. (Mathematically it would mean any sequence of characters from 0 to infinity in length that weren't these two in a row, and that is likely not of any real use.)

(F) The two-character sequence "(*" in this context in a regular expression pattern should be an indivisible token, with nothing intervening between the "(" and the "*", but you separated them.

(F) Your machine apparently doesn't implement ioctl(), which is pretty strange for a machine that supports C.
(W unopened) You tried ioctl() on a filehandle that was never opened. Check your control flow and number of arguments.
(F) Your Perl has not been configured to have PerlIO, and therefore you cannot use IO layers. To have PerlIO, Perl must be configured with 'useperlio'.
(F) Your machine doesn't implement the sockatmark() functionality, neither as a system call nor an ioctl call (SIOCATMARK).
'%s' is an unknown bound type in regex; marked by <-- HERE in m/%s/
(F) You used "\b{...}" or "\B{...}" and the "..." is not known to Perl. The current valid ones are given in "\b{}, \b, \B{}, \B" in perlrebackslash.
%s() isn't allowed on :utf8 handles
(F) The sysread(), recv(), syswrite() and send() operators are not allowed on handles that have the ":utf8" layer, either explicitly, or implicitly, eg., with the :encoding(UTF-16LE) layer.

Previously sysread() and recv() currently use only the ":utf8" flag for the stream, ignoring the actual layers. Since sysread() and recv() did no UTF-8 validation they can end up creating invalidly encoded scalars.

Similarly, syswrite() and send() used only the ":utf8" flag, otherwise ignoring any layers. If the flag is set, both wrote the value UTF-8 encoded, even if the layer is some different encoding, such as the example above.

Ideally, all of these operators would completely ignore the ":utf8" state, working only with bytes, but this would result in silently breaking existing code.

"%s" is more clearly written simply as "%s" in regex; marked by <-- HERE in m/%s/
(W regexp) (only under "use re 'strict'" or within "(?[...])")

You specified a character that has the given plainer way of writing it, and which is also portable to platforms running with different character sets.

$* is no longer supported as of Perl 5.30
(F) The special variable $*, deprecated in older perls, was removed in 5.10.0, is no longer supported and is a fatal error as of Perl 5.30. In previous versions of perl the use of $* enabled or disabled multi-line matching within a string.

Instead of using $* you should use the "/m" (and maybe "/s") regexp modifiers. You can enable "/m" for a lexical scope (even a whole file) with "use re '/m'". (In older versions: when $* was set to a true value then all regular expressions behaved as if they were written using "/m".)

Use of this variable will be a fatal error in Perl 5.30.

$# is no longer supported as of Perl 5.30
(F) The special variable $#, deprecated in older perls, was removed as of 5.10.0, is no longer supported and is a fatal error as of Perl 5.30. You should use the printf/sprintf functions instead.
'%s' is not a code reference
(W overload) The second (fourth, sixth, ...) argument of overload::constant needs to be a code reference. Either an anonymous subroutine, or a reference to a subroutine.
'%s' is not an overloadable type
(W overload) You tried to overload a constant type the overload package is unaware of.
'%s' is not recognised as a builtin function
(F) An attempt was made to "use" the builtin pragma module to create a lexical alias for an unknown function name.
(S inplace) The "-i" option was passed on the command line, indicating that the script is intended to edit files in place, but no files were given. This is usually a mistake, since editing STDIN in place doesn't make sense, and can be confusing because it can make perl look like it is hanging when it is really just trying to read from STDIN. You should either pass a filename to edit, or remove "-i" from the command line. See perlrun for more details.
(P) The regular expression parser is confused.
(F) Your regular expression used "\K" in a lookahead or lookbehind assertion, which currently isn't permitted.

This may change in the future, see Support \K in lookarounds <https://github.com/Perl/perl5/issues/18134>.

(F) You named a loop to break out of, but you're not currently in a loop of that name, not even if you count where you were called from. See "last" in perlfunc.
(F) You named a loop to continue, but you're not currently in a loop of that name, not even if you count where you were called from. See "last" in perlfunc.
(F) You named a loop to restart, but you're not currently in a loop of that name, not even if you count where you were called from. See "last" in perlfunc.
(F) While under the "use filetest" pragma, switching the real and effective uids or gids failed.
(F) While unpacking, the string buffer was already used up when an unpack length/code combination tried to obtain more data. This results in an undefined value for the length. See "pack" in perlfunc.
(W syntax) You used length() on either an array or a hash when you probably wanted a count of the items.

Array size can be obtained by doing:

    scalar(@array);

The number of items in a hash can be obtained by doing:

    scalar(keys %hash);
(F) An extension is attempting to insert text into the current parse (using lex_stuff_pvn or similar), but tried to insert a character that couldn't be part of the current input. This is an inherent pitfall of the stuffing mechanism, and one of the reasons to avoid it. Where it is necessary to stuff, stuffing only plain ASCII is recommended.
(F) Lexing code supplied by an extension violated the lexer's API in a detectable way.
(W closed) You tried to do a listen on a closed socket. Did you forget to check the return value of your socket() call? See "listen" in perlfunc.
(F) On some platforms, notably Windows, the three-or-more-arguments form of "open" does not support pipes, such as "open($pipe, '|-', @args)". Use the two-argument "open($pipe, '|prog arg1 arg2...')" form instead.
(F) (only under "use re 'strict'" or within "(?[...])")

Likely you forgot the "/x" modifier or there was a typo in the pattern. For example, did you really mean to match a form-feed? If so, all the ASCII vertical space control characters are representable by escape sequences which won't present such a jarring appearance as your pattern does when displayed.

  \r    carriage return
  \f    form feed
  \n    line feed
  \cK   vertical tab
%s: loadable library and perl binaries are mismatched (got %s handshake key %p, needed %p)
(P) A dynamic loading library ".so" or ".dll" was being loaded into the process that was built against a different build of perl than the said library was compiled against. Reinstalling the XS module will likely fix this error.
(W locale) You are using the named UTF-8 locale. UTF-8 locales are expected to have very particular behavior, which most do. This message arises when perl found some departures from the expectations, and is notifying you that the expected behavior overrides these differences. In some cases the differences are caused by the locale definition being defective, but the most common causes of this warning are when there are ambiguities and conflicts in following the Standard, and the locale has chosen an approach that differs from Perl's.

One of these is because that, contrary to the claims, Unicode is not completely locale insensitive. Turkish and some related languages have two types of "I" characters. One is dotted in both upper- and lowercase, and the other is dotless in both cases. Unicode allows a locale to use either the Turkish rules, or the rules used in all other instances, where there is only one type of "I", which is dotless in the uppercase, and dotted in the lower. The perl core does not (yet) handle the Turkish case, and this message warns you of that. Instead, the Unicode::Casing module allows you to mostly implement the Turkish casing rules.

The other common cause is for the characters

 $ + < = > ^ ` | ~

These are problematic. The C standard says that these should be considered punctuation in the C locale (and the POSIX standard defers to the C standard), and Unicode is generally considered a superset of the C locale. But Unicode has added an extra category, "Symbol", and classifies these particular characters as being symbols. Most UTF-8 locales have them treated as punctuation, so that ispunct(3) returns non-zero for them. But a few locales have it return 0. Perl takes the first approach, not using ispunct() at all (see Note [5] in perlrecharclass), and this message is raised to notify you that you are getting Perl's approach, not the locale's.

(S locale) The named locale is not supported by Perl, and using it leads to undefined behavior, including potentially crashing the computer.

Currently the only locales that generate this severe warning are non-UTF-8 ones which have characters that require more than one byte to represent (common in older East Asian language locales). See perllocale.

(W locale) You are using the named locale, which is a non-UTF-8 one, and which perl has determined is not fully compatible with what it can handle. The second %s gives a reason.

By far the most common reason is that the locale has characters in it that are represented by more than one byte. The only such locales that Perl can handle are the UTF-8 locales. Most likely the specified locale is a non-UTF-8 one for an East Asian language such as Chinese or Japanese. If the locale is a superset of ASCII, the ASCII portion of it may work in Perl.

Some essentially obsolete locales that aren't supersets of ASCII, mainly those in ISO 646 or other 7-bit locales, such as ASMO 449, can also have problems, depending on what portions of the ASCII character set get changed by the locale and are also used by the program. The warning message lists the determinable conflicting characters.

Note that not all incompatibilities are found.

If this happens to you, there's not much you can do except switch to use a different locale or use Encode to translate from the locale into UTF-8; if that's impracticable, you have been warned that some things may break.

This message is output once each time a bad locale is switched into within the scope of "use locale", or on the first possibly-affected operation if the "use locale" inherits a bad one. It is not raised for any operations from the POSIX module.

(W overflow) You called "localtime" with a number that it could not handle: too large, too small, or NaN. The returned value is "undef".
(W overflow) You called "localtime" with a number that was larger than it can reliably handle and "localtime" probably returned the wrong date. This warning is also triggered with NaN (the special not-a-number value).
(W overflow) You called "localtime" with a number that was smaller than it can reliably handle and "localtime" probably returned the wrong date.
(F) There is currently a limit on the length of string which lookbehind can handle. This restriction may be eased in a future release.
(W imprecision) You attempted to increment or decrement a value by one, but the result is too large for the underlying floating point representation to store accurately. Hence, the target of "++" or "--" is increased or decreased by quite different value than one, such as zero (i.e. the target is unchanged) or two, due to rounding. Perl issues this warning because it has already switched from integers to floating point when values are too large for integers, and now even floating point is insufficient. You may wish to switch to using Math::BigInt explicitly.
(W io) You tried to do an lstat on a filehandle. What did you mean by that? lstat() makes sense only on filenames. (Perl did a fstat() instead on the filehandle.)
(W misc) Although attributes.pm allows this, turning the lvalue attribute on or off on a Perl subroutine that is already defined does not always work properly. It may or may not do what you want, depending on what code is inside the subroutine, with exact details subject to change between Perl versions. Only do this if you really know what you are doing.
(W misc) Using the ":lvalue" declarative syntax to make a Perl subroutine an lvalue subroutine after it has been defined is not permitted. To make the subroutine an lvalue subroutine, add the lvalue attribute to the definition, or put the "sub foo :lvalue;" declaration before the definition.

See also attributes.pm.

(F) You assigned a magical array to a stash element, and then tried to use the subroutine from the same slot. You are asking Perl to do something it cannot do, details subject to change between Perl versions.
(F) Between the brackets enclosing a numeric repeat count only digits are permitted. See "pack" in perlfunc.
(F) Between the brackets enclosing a numeric repeat count only digits are permitted. See "pack" in perlfunc.
(F) An error peculiar to OS/2. PERLLIB_PREFIX should be of the form 

    prefix1;prefix2

or
prefix1 prefix2

with nonempty prefix1 and prefix2. If "prefix1" is indeed a prefix of a builtin library search path, prefix2 is substituted. The error may appear if components are not found, or are too long. See "PERLLIB_PREFIX" in perlos2.

(F) You tried to use a function with a malformed prototype. The syntax of function prototypes is given a brief compile-time check for obvious errors like invalid characters. A more rigorous check is run when the function is called. Perhaps the function's author was trying to write a subroutine signature but didn't enable that feature first ("use feature 'signatures'"), so the signature was instead interpreted as a bad prototype.
(S utf8)(F) Perl detected a string that should be UTF-8, but didn't comply with UTF-8 encoding rules, or represents a code point whose ordinal integer value doesn't fit into the word size of the current platform (overflows). Details as to the exact malformation are given in the variable, %s, part of the message.

One possible cause is that you set the UTF8 flag yourself for data that you thought to be in UTF-8 but it wasn't (it was for example legacy 8-bit data). To guard against this, you can use "Encode::decode('UTF-8', ...)".

If you use the :encoding(UTF-8) PerlIO layer for input, invalid byte sequences are handled gracefully, but if you use ":utf8", the flag is set without validating the data, possibly resulting in this error message.

See also "Handling Malformed Data" in Encode.

(F) The charnames handler returned malformed UTF-8.
(F) This message indicates a bug either in the Perl core or in XS code. Such code was trying to find out if a character, allegedly stored internally encoded as UTF-8, was of a given type, such as being punctuation or a digit. But the character was not encoded in legal UTF-8. The %s is replaced by a string that can be used by knowledgeable people to determine what the type being checked against was.

Passing malformed strings was deprecated in Perl 5.18, and became fatal in Perl 5.26.

(F) You tried to unpack something that didn't comply with UTF-8 encoding rules and perl was unable to guess how to make more progress.
(F) You tried to pack something that didn't comply with UTF-8 encoding rules and perl was unable to guess how to make more progress.
(F) You tried to unpack something that didn't comply with UTF-8 encoding rules and perl was unable to guess how to make more progress.
(F) Perl thought it was reading UTF-16 encoded character data but while doing it Perl met a malformed Unicode surrogate.
(F) In a subroutine signature, you wrote something like "$x = undef, $y", making an earlier parameter optional and a later one mandatory. Parameters are filled from left to right, so it's impossible for the caller to omit an earlier one and pass a later one. If you want to act as if the parameters are filled from right to left, declare the rightmost optional and then shuffle the parameters around in the subroutine's body.
(S non_unicode) Perl allows strings to contain a superset of Unicode code points; each code point may be as large as what is storable in a signed integer on your system, but these may not be accepted by other languages/systems. This message occurs when you matched a string containing such a code point against a regular expression pattern, and the code point was matched against a Unicode property, "\p{...}" or "\P{...}". Unicode properties are only defined on Unicode code points, so the result of this match is undefined by Unicode, but Perl (starting in v5.20) treats non-Unicode code points as if they were typical unassigned Unicode ones, and matched this one accordingly. Whether a given property matches these code points or not is specified in "Properties accessible through \p{} and \P{}" in perluniprops.

This message is suppressed (unless it has been made fatal) if it is immaterial to the results of the match if the code point is Unicode or not. For example, the property "\p{ASCII_Hex_Digit}" only can match the 22 characters "[0-9A-Fa-f]", so obviously all other code points, Unicode or not, won't match it. (And "\P{ASCII_Hex_Digit}" will match every code point except these 22.)

Getting this message indicates that the outcome of the match arguably should have been the opposite of what actually happened. If you think that is the case, you may wish to make the "non_unicode" warnings category fatal; if you agree with Perl's decision, you may wish to turn off this category.

See "Beyond Unicode code points" in perlunicode for more information.

%s matches null string many times in regex; marked by <-- HERE in m/%s/
(W regexp) The pattern you've specified would be an infinite loop if the regular expression engine didn't specifically check for that. The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.
(F) Perl aborted due to too high a number of signals pending. This usually indicates that your operating system tried to deliver signals too fast (with a very high priority), starving the perl process from resources it would need to reach a point where it can process signals safely. (See "Deferred Signals (Safe Signals)" in perlipc.)
"%s" may clash with future reserved word
(W) This warning may be due to running a perl5 script through a perl4 interpreter, especially if the word that is being warned about is "use" or "my".
'%' may not be used in pack
(F) You can't pack a string by supplying a checksum, because the checksumming process loses information, and you can't go the other way. See "unpack" in perlfunc.
(F) An attempt was made to specify an entry in an overloading table that doesn't resolve to a valid subroutine. See overload.
(S experimental::class) This warning is emitted if you use the "method" keyword of "use feature 'class'". This keyword is currently experimental and its behaviour may change in future releases of Perl.
See "500 Server error".
(W redefine) You redefined a method. To suppress this warning, say 

    {
        no warnings 'redefine';
        *name = method { ... };
    }
(S) An advisory indicating that the previous error may have been caused by a missing delimiter on a string or pattern, because it eventually ended earlier on the current line.
(F) A pack template could not be parsed because pairs of "[...]" or "(...)" could not be matched up. See "pack" in perlfunc.
(W syntax) An underscore (underbar) in a numeric constant did not separate two digits.
(F) A %n was used in a format string with no corresponding argument for perl to write the current string length to.
(W missing) You called a function with fewer arguments than other arguments you supplied indicated would be needed.

Currently only emitted when a printf-type format required more arguments than were supplied, but might be used in the future for other cases where we can statically determine that arguments to functions are missing, e.g. for the "pack" in perlfunc function.

(F) The argument to the indicated command line switch must follow immediately after the switch, without intervening spaces.
(F) Wrong syntax of character name literal "\N{charname}" within double-quotish context. This can also happen when there is a space (or comment) between the "\N" and the "{" in a regex with the "/x" modifier. This modifier does not change the requirement that the brace immediately follow the "\N".
(F) A "\o" must be followed immediately by a "{" in double-quotish context.
(F) While certain functions allow you to specify a filehandle or an "indirect object" before the argument list, this ain't one of them.
(F) While certain operators allow you to specify a filehandle or an "indirect object" before the argument list, "return" isn't one of them.
(W pipe) You used the "open(FH, "| command")" or "open(FH, "command |")" construction, but the command was missing or blank.
(F) A double-quoted string ended with "\c", without the required control character name.
(W illegalproto) A grouping was started with "[" but never closed with "]".
(F) The syntax for lexically scoped subroutines requires that they have a name with which they can be found.
(F) Apparently you've been programming in csh too much. Variables are always mentioned with the $ in Perl, unlike in the shells, where it can vary from one line to the next.
(Missing operator before %s?)
(S syntax) This is an educated guess made in conjunction with the message "%s found where operator expected". Often the missing operator is a comma.
(F) You tried to call "require" or "do" with no argument or with an undefined value as an argument. Require expects either a package name or a file-specification as an argument; do expects a filename. See "require EXPR" in perlfunc and "do EXPR" in perlfunc.
(F) A "%{^HOOK}{require__before}" hook rewrote the name of the file being compiled with "require" or "do" with an empty string an undefined value which is forbidden. See "%{^HOOK}" in perlvar and "require EXPR" in perlfunc.
(F) Missing right brace in "\x{...}", "\p{...}", "\P{...}", or "\N{...}".
(F) "\N" has two meanings.

The traditional one has it followed by a name enclosed in braces, meaning the character (or sequence of characters) given by that name. Thus "\N{ASTERISK}" is another way of writing "*", valid in both double-quoted strings and regular expression patterns. In patterns, it doesn't have the meaning an unescaped "*" does.

Starting in Perl 5.12.0, "\N" also can have an additional meaning (only) in patterns, namely to match a non-newline character. (This is short for "[^\n]", and like "." but is not affected by the "/s" regex modifier.)

This can lead to some ambiguities. When "\N" is not followed immediately by a left brace, Perl assumes the "[^\n]" meaning. Also, if the braces form a valid quantifier such as "\N{3}" or "\N{5,}", Perl assumes that this means to match the given quantity of non-newlines (in these examples, 3; and 5 or more, respectively). In all other case, where there is a "\N{" and a matching "}", Perl assumes that a character name is desired.

However, if there is no matching "}", Perl doesn't know if it was mistakenly omitted, or if "[^\n]{" was desired, and raises this error. If you meant the former, add the right brace; if you meant the latter, escape the brace with a backslash, like so: "\N\{"

(F) The lexer counted more opening curly or square brackets than closing ones. As a general rule, you'll find it's missing near the place you were last editing.
(Missing semicolon on previous line?)
(S syntax) This is an educated guess made in conjunction with the message "%s found where operator expected". Don't automatically put a semicolon on the previous line just because you saw this message.
(F) You tried, directly or indirectly, to change the value of a constant. You didn't, of course, try "2 = 1", because the compiler catches that. But an easy way to do the same thing is: 

    sub mod { $_[0] = 1 }
    mod(2);

Another way is to assign to a substr() that's off the end of the string.

Yet another way is to assign to a "foreach" loop VAR when VAR is aliased to a constant in the look LIST:

    $x = 1;
    foreach my $n ($x, 2) {
        $n *= 2; # modifies the $x, but fails on attempt to
    }            # modify the 2

PerlIO::scalar will also produce this message as a warning if you attempt to open a read-only scalar for writing.

(F) You tried to make an array value spring into existence, and the subscript was probably negative, even counting from end of the array backwards.
(P) You tried to make a hash value spring into existence, and it couldn't be created for some peculiar reason.
(F) Only a bare module name is allowed as the first argument to a "use".
(F) The "-M" or "-m" options say that Perl should load some module, but you omitted the name of the module. Consult perlrun for full details about "-M" and "-m".
(F) The "open" function has been asked to open multiple files. This can happen if you are trying to open a pipe to a command that takes a list of arguments, but have forgotten to specify a piped open mode. See "open" in perlfunc for details.
(S) You compiled perl with -DPERL_DEBUG_READONLY_COW (see "Copy on Write" in perlguts), but a shared string buffer could not be made read-only.
(S) You compiled perl with -DPERL_DEBUG_READONLY_OPS (see perlhacktips), but an op tree could not be made read-only.
(S) You compiled perl with -DPERL_DEBUG_READONLY_COW (see "Copy on Write" in perlguts), but a read-only shared string buffer could not be made mutable.
(S) You compiled perl with -DPERL_DEBUG_READONLY_OPS (see perlhacktips), but a read-only op tree could not be made mutable before freeing the ops.
(F) You don't have System V message IPC on your system.
(F) You supplied a list of subscripts to a hash lookup under "no feature "multidimensional";", eg: 

  $z = $foo{$x, $y};

which by default acts like:

  $z = $foo{join($;, $x, $y)};
(W syntax) Multidimensional arrays aren't written like $foo[1,2,3]. They're written like "$foo[1][2][3]", as in C.
(F) In subroutine signatures, a slurpy parameter ("@" or "%") must be the last parameter, and there must not be more than one of them; for example: 

    sub foo ($x, @y)    {} # legal
    sub foo ($x, @y, %) {} # invalid
'/' must follow a numeric type in unpack
(F) You had an unpack template that contained a '/', but this did not follow some unpack specification producing a numeric value. See "pack" in perlfunc.
%s must not be a named sequence in transliteration operator
(F) Transliteration ("tr///" and "y///") transliterates individual characters. But a named sequence by definition is more than an individual character, and hence doing this operation on it doesn't make sense.
"my sub" not yet implemented
(F) Lexically scoped subroutines are not yet implemented. Don't try that yet.
"my" subroutine %s can't be in a package
(F) Lexically scoped subroutines aren't in a package, so it doesn't make sense to try to declare one with a package qualifier on the front.
"my %s" used in sort comparison
(W syntax) The package variables $a and $b are used for sort comparisons. You used $a or $b in as an operand to the "<=>" or "cmp" operator inside a sort comparison block, and the variable had earlier been declared as a lexical variable. Either qualify the sort variable with the package name, or rename the lexical variable.
"my" variable %s can't be in a package
(F) Lexically scoped variables aren't in a package, so it doesn't make sense to try to declare one with a package qualifier on the front. Use local() if you want to localize a package variable.
(W once) Typographical errors often show up as unique variable names. If you had a good reason for having a unique name, then just mention it again somehow to suppress the message. The "our" declaration is also provided for this purpose.

NOTE: This warning detects package symbols that have been used only once. This means lexical variables will never trigger this warning. It also means that all of the package variables $c, @c, %c, as well as *c, &c, sub c{}, c(), and c (the filehandle or format) are considered the same; if a program uses $c only once but also uses any of the others it will not trigger this warning. Symbols beginning with an underscore and symbols using special identifiers (q.v. perldata) are exempt from this warning.

(F) Within "(?[   ])", all constants interpreted as octal need to be exactly 3 digits long. This helps catch some ambiguities. If your constant is too short, add leading zeros, like 

 (?[ [ \078 ] ])     # Syntax error!
 (?[ [ \0078 ] ])    # Works
 (?[ [ \007 8 ] ])   # Clearer

The maximum number this construct can express is "\777". If you need a larger one, you need to use \o{} instead. If you meant two separate things, you need to separate them:

 (?[ [ \7776 ] ])        # Syntax error!
 (?[ [ \o{7776} ] ])     # One meaning
 (?[ [ \777 6 ] ])       # Another meaning
 (?[ [ \777 \006 ] ])    # Still another
(F) The length count obtained from a length/code unpack operation was negative. See "pack" in perlfunc.
(F) You tried to do a read/write/send/recv operation with a buffer length that is less than 0. This is difficult to imagine.
(F) When "vec" is called in an lvalue context, the second argument must be greater than or equal to zero.
(W numeric) You tried to execute the "x" repetition operator fewer than 0 times, which doesn't make sense.
(F) You can't quantify a quantifier without intervening parentheses. So things like ** or +* or ?* are illegal. The <-- HERE shows whereabouts in the regular expression the problem was discovered.

Note that the minimal matching quantifiers, "*?", "+?", and "??" appear to be nested quantifiers, but aren't. See perlre.

%s never introduced
(S internal) The symbol in question was declared but somehow went out of scope before it could possibly have been used.
(F) "next::method" needs to be called within the context of a real method in a real package, and it could not find such a context. See mro.
(F) The new (as of Perl 5.12) meaning of "\N" as "[^\n]" is not valid in a bracketed character class, for the same reason that "." in a character class loses its specialness: it matches almost everything, which is probably not what you want.
(F) Named Unicode character escapes ("\N{...}") may return a multi-character sequence. Even though a character class is supposed to match just one character of input, perl will match the whole thing correctly, except under certain conditions. These currently are
The mathematically logical behavior for what matches when inverting is very different from what people expect, so we have decided to forbid it.
Similarly unclear is what should be generated when the "\N{...}" is used as one of the end points of the range, such as in 

 [\x{41}-\N{ARABIC SEQUENCE YEH WITH HAMZA ABOVE WITH AE}]

What is meant here is unclear, as the "\N{...}" escape is a sequence of code points, so this is made an error.

The syntax "(?[   ])" in a regular expression yields a list of single code points, none can be a sequence.
(F) Certain operations are deemed to be too insecure for a setuid or setgid script to even be allowed to attempt. Generally speaking there will be another way to do what you want that is, if not secure, at least securable. See perlsec.
(F) Perl's -e and -E command-line options require an argument. If you want to run an empty program, pass the empty string as a separate argument or run a program consisting of a single 0 or 1: 

    perl -e ""
    perl -e0
    perl -e1
(F) A list operator that has a filehandle or "indirect object" is not allowed to have a comma between that and the following arguments. Otherwise it'd be just another one of the arguments.

One possible cause for this is that you expected to have imported a constant to your name space with use or import while no such importing took place, it may for example be that your operating system does not support that particular constant. Hopefully you did use an explicit import list for the constants you expect to see; please see "use" in perlfunc and "import" in perlfunc. While an explicit import list would probably have caught this error earlier it naturally does not remedy the fact that your operating system still does not support that constant. Maybe you have a typo in the constants of the symbol import list of use or import or in the constant name at the line where this error was triggered?

(F) An error peculiar to VMS. Perl handles its own command line redirection, and found a '|' at the end of the command line, so it doesn't know where you want to pipe the output from this command.
(F) The currently executing code was compiled with the -d switch, but for some reason the current debugger (e.g. perl5db.pl or a "Devel::" module) didn't define a routine to be called at the beginning of each statement.
(P) This is counted as an internal error, because every machine should supply dbm nowadays, because Perl comes with SDBM. See SDBM_File.
(F) The currently executing code was compiled with the -d switch, but for some reason the current debugger (e.g. perl5db.pl or a "Devel::" module) didn't define a "DB::sub" routine to be called at the beginning of each ordinary subroutine call.
(F) No hexadecimal digits were found following "0x" or no binary digits were found following "0b".
(F) The -I command-line switch requires a directory name as part of the same argument. Use -Ilib, for instance. -I lib won't work.
(F) An error peculiar to VMS. Perl handles its own command line redirection, and found a '2>' or a '2>>' on the command line, but can't find the name of the file to which to write data destined for stderr.
(F) A pack or unpack template has an opening '(' or '[' without its matching counterpart. See "pack" in perlfunc.
(F) An error peculiar to VMS. Perl handles its own command line redirection, and found a '<' on the command line, but can't find the name of the file from which to read data for stdin.
(F) "next::method" found no further instances of this method name in the remaining packages of the MRO of this class. If you don't want it throwing an exception, use "maybe::next::method" or "next::can". See mro.
(W numeric) You tried to execute the "x" repetition operator "Inf" (or "-Inf") or "NaN" times, which doesn't make sense.
(F) In a regular expression, there was a non-hexadecimal character where a hex one was expected, like 

 (?[ [ \xDG ] ])
 (?[ [ \x{DEKA} ] ])
(W digit) In parsing a hexadecimal numeric constant, a character was unexpectedly encountered that isn't hexadecimal. The resulting value is as indicated.

Note that, within braces, every character starting with the first non-hexadecimal up to the ending brace is ignored.

(F) In a regular expression, there was a non-octal character where an octal one was expected, like 

 (?[ [ \o{1278} ] ])
(W digit) In parsing an octal numeric constant, a character was unexpectedly encountered that isn't octal. The resulting value is as indicated.

When not using "\o{...}", you wrote something like "\08", or "\179" in a double-quotish string. The resolution is as indicated, with all but the last digit treated as a single character, specified in octal. The last digit is the next character in the string. To tell Perl that this is indeed what you want, you can use the "\o{ }" syntax, or use exactly three digits to specify the octal for the character.

Note that, within braces, every character starting with the first non-octal up to the ending brace is ignored.

"no" not allowed in expression
(F) The "no" keyword is recognized and executed at compile time, and returns no useful value. See perlmod.
(W misc) A number has been passed as a bitmask argument to select(). Use the vec() function to construct the file descriptor bitmasks for select. See "select" in perlfunc.
(F) An error peculiar to VMS. Perl handles its own command line redirection, and found a lone '>' at the end of the command line, so it doesn't know where you wanted to redirect stdout.
(F) An error peculiar to VMS. Perl handles its own command line redirection, and found a '>' or a '>>' on the command line, but can't find the name of the file to which to write data destined for stdout.
(F) Fully qualified subroutine and variable names are not allowed in "our" declarations, because that doesn't make much sense under existing rules. Such syntax is reserved for future extensions.
(F) You called "perl -x", but no line was found in the file beginning with #! and containing the word "perl".
(F) Configure didn't find anything resembling the setregid() call for your system.
(F) Configure didn't find anything resembling the setreuid() call for your system.
(F) You provided a class qualifier in a "my", "our" or "state" declaration, but this class doesn't exist at this point in your program.
(F) You tried to access a key from a hash through the indicated typed variable but that key is not allowed by the package of the same type. The indicated package has restricted the set of allowed keys using the fields pragma.
(F) You specified a signal hook that was not recognized by Perl. Currently, Perl accepts "__DIE__" and "__WARN__" as valid signal hooks.
(P) An error peculiar to VMS. The internal routine my_pclose() tried to close a pipe which hadn't been opened. This should have been caught earlier as an attempt to close an unopened filehandle.
(W signal) You specified a signal name as a subscript to %SIG that was not recognized. Say "kill -l" in your shell to see the valid signal names on your system.
(W regexp) You specified a wildcard for a Unicode property value, but there is no property value in the current Unicode release that matches it. Check your spelling.
(F) Perl was trying to evaluate a reference to a code value (that is, a subroutine), but found a reference to something else instead. You can use the ref() function to find out what kind of ref it really was. See also perlref.
(F) Perl was trying to evaluate a reference to a "typeglob" (that is, a symbol table entry that looks like *foo), but found a reference to something else instead. You can use the ref() function to find out what kind of ref it really was. See perlref.
(F) Perl was trying to evaluate a reference to a hash value, but found a reference to something else instead. You can use the ref() function to find out what kind of ref it really was. See perlref.
'#' not allowed immediately following a sigil in a subroutine signature
(F) In a subroutine signature definition, a comment following a sigil ("$", "@" or "%"), needs to be separated by whitespace or a comma etc., in particular to avoid confusion with the $# variable. For example: 

    # bad
    sub f ($# ignore first arg
           , $y) {}
    # good
    sub f ($, # ignore first arg
           $y) {}
(F) Perl was trying to evaluate a reference to an array value, but found a reference to something else instead. You can use the ref() function to find out what kind of ref it really was. See perlref.
(F) Perl was trying to evaluate a reference to a scalar value, but found a reference to something else instead. You can use the ref() function to find out what kind of ref it really was. See perlref.
(F) Perl was trying to evaluate a reference to a code value (that is, a subroutine), but found a reference to something else instead. You can use the ref() function to find out what kind of ref it really was. See also perlref.
(F) An attempt was made to specify an entry in an overloading table that doesn't somehow point to a valid subroutine. See overload.
(F) The function requires more arguments than you specified.
(W syntax) A format specified more picture fields than the next line supplied. See perlform.
%s: not found
(A) You've accidentally run your script through the Bourne shell instead of Perl. Check the #! line, or manually feed your script into Perl yourself.
(S) A warning peculiar to VMS. Perl was unable to find the local timezone offset, so it's assuming that local system time is equivalent to UTC. If it's not, define the logical name SYS$TIMEZONE_DIFFERENTIAL to translate to the number of seconds which need to be added to UTC to get local time.
(S debugging) Some internal routine called run() with a null opcode pointer.
(F) The first argument to formline must be a valid format picture specification. It was found to be empty, which probably means you supplied it an uninitialized value. See perlform.
(P) The internal pattern matching routines are out of their gourd.
(F) Perl limits the representation of decimal numbers in programs to about 250 characters. You've exceeded that length. Future versions of Perl are likely to eliminate this arbitrary limitation. In the meantime, try using scientific notation (e.g. "1e6" instead of "1_000_000").
(F) Perl was looking for a number but found nothing that looked like a number. This happens, for example with "\o{}", with no number between the braces.
(F) The length of the result of a numeric format supplied to sprintf() or printf() would have been too large for the underlying C function to report. This limit is typically 2GB.
(F) The only numeric variable which is allowed to start with a 0 is $0, and you mentioned a variable that starts with 0 that has more than one digit. You probably want to remove the leading 0, or if the intent was to express a variable name in octal you should convert to decimal.
(W portable) The octal number you specified is larger than 2**32-1 (4294967295) and therefore non-portable between systems. See perlport for more on portability concerns.
(F) A subroutine using a slurpy hash parameter in its signature received an odd number of arguments to populate the hash. It requires the arguments to be paired, with the same number of keys as values. The caller of the subroutine is presumably at fault.

The message attempts to include the name of the called subroutine. If the subroutine has been aliased, the subroutine's original name will be shown, regardless of what name the caller used.

(W overload) The call to overload::constant contained an odd number of arguments. The arguments should come in pairs.
(S) The constructor for the given class was called with an odd number of arguments.

Class constructors accept named arguments as a list and expect pairs of name followed by value as if you were initialising a hash.

(W misc) You specified an odd number of elements to initialize a hash, which is odd, because hashes come in key/value pairs.
(F) A call to "export_lexically" in builtin contained an odd number of arguments. This is not permitted, because each name must be paired with a valid reference value.
(W misc) You specified an odd number of elements to initialize a hash, which is odd, because hashes come in key/value pairs.
(W misc) You specified an odd number of elements to initialise a hash field of an object. Hashes are initialised from a list of key/value pairs so there must be a corresponding value to every key. The final missing value will be filled in with undef instead.
(W layer) You tried to do a read/write/send/recv/seek operation with an offset pointing outside the buffer. This is difficult to imagine. The sole exceptions to this are that zero padding will take place when going past the end of the string when either sysread()ing a file, or when seeking past the end of a scalar opened for I/O (in anticipation of future reads and to imitate the behavior with real files).
(D deprecated::apostrophe_as_package_separator, syntax) You used the old package separator "'" in a variable, subroutine or package name. Support for the old package separator will be removed in Perl 5.42.
(D deprecated::apostrophe_as_package_separator, syntax) You used the old package separator, "'", in a variable named inside a double-quoted string; e.g., "In $name's house". This is equivalent to "In $name::s house". If you meant the former, put a backslash before the apostrophe ("In $name\'s house").

Support for the old package separator will be removed in Perl 5.42.

(F) You tried to apply the ":param" attribute to an array or hash field. Currently this is not permitted.
%s() on unopened %s
(W unopened) An I/O operation was attempted on a filehandle that was never initialized. You need to do an open(), a sysopen(), or a socket() call, or call a constructor from the FileHandle package.
-%s on unopened filehandle %s
(W unopened) You tried to invoke a file test operator on a filehandle that isn't open. Check your control flow. See also "-X" in perlfunc.
(S internal) An internal warning that the grammar is screwed up.
(S internal) An internal warning that the grammar is screwed up.
(F) You wrote something like 

 (?[ \p{Digit} \p{Thai} ])

There are two operands, but no operator giving how you want to combine them.

(F) An attempt was made to perform an overloaded operation for which no handler was defined. While some handlers can be autogenerated in terms of other handlers, there is no default handler for any operation, unless the "fallback" overloading key is specified to be true. See overload.
(S non_unicode) You performed an operation requiring Unicode rules on a code point that is not in Unicode, so what it should do is not defined. Perl has chosen to have it do nothing, and warn you.

If the operation shown is "ToFold", it means that case-insensitive matching in a regular expression was done on the code point.

If you know what you are doing you can turn off this warning by "no warnings 'non_unicode';".

(S surrogate) You performed an operation requiring Unicode rules on a Unicode surrogate. Unicode frowns upon the use of surrogates for anything but storing strings in UTF-16, but rules are (reluctantly) defined for the surrogates, and they are to do nothing for this operation. Because the use of surrogates can be dangerous, Perl warns.

If the operation shown is "ToFold", it means that case-insensitive matching in a regular expression was done on the code point.

If you know what you are doing you can turn off this warning by "no warnings 'surrogate';".

(S ambiguous) You used a variable or subroutine call where the parser was expecting an operator. The parser has assumed you really meant to use an operator, but this is highly likely to be incorrect. For example, if you say "*foo *foo" it will be interpreted as if you said "*foo * 'foo'".
(F) In a subroutine signature, you wrote something like "$x =", making a named optional parameter without a default value. A nameless optional parameter is permitted to have no default value, but a named one must have a specific default. You probably want "$x = undef".
"our" variable %s redeclared
(W shadow) You seem to have already declared the same global once before in the current lexical scope.
(X) The malloc() function returned 0, indicating there was insufficient remaining memory (or virtual memory) to satisfy the request. Perl has no option but to exit immediately.

At least in Unix you may be able to get past this by increasing your process datasize limits: in csh/tcsh use "limit" and "limit datasize n" (where "n" is the number of kilobytes) to check the current limits and change them, and in ksh/bash/zsh use "ulimit -a" and "ulimit -d n", respectively.

(X) An attempt was made to extend an array, a list, or a string beyond the largest possible memory allocation.
(F) The malloc() function returned 0, indicating there was insufficient remaining memory (or virtual memory) to satisfy the request. However, the request was judged large enough (compile-time default is 64K), so a possibility to shut down by trapping this error is granted.
(X)(F) The malloc() function returned 0, indicating there was insufficient remaining memory (or virtual memory) to satisfy the request.

The request was judged to be small, so the possibility to trap it depends on the way perl was compiled. By default it is not trappable. However, if compiled for this, Perl may use the contents of $^M as an emergency pool after die()ing with this message. In this case the error is trappable once, and the error message will include the line and file where the failed request happened.

(F) You can't allocate more than 2^31+"small amount" bytes. This error is most likely to be caused by a typo in the Perl program. e.g., $arr[time] instead of $arr[$time].
(F) An attempt was made to extend a string beyond the largest possible memory allocation by assigning to vec() called with a large second argument.
(F) The yacc parser wanted to grow its stack so it could continue parsing, but realloc() wouldn't give it more memory, virtual or otherwise.
(X) A low-level memory allocation routine failed, indicating there was insufficient remaining (virtual) memory to satisfy the request. Perl has no option but to exit immediately.

At least in Unix you may be able to get past this by increasing your process datasize limits: in csh/tcsh use "limit" and "limit datasize n" (where "n" is the number of kilobytes) to check the current limits and change them, and in ksh/bash/zsh use "ulimit -a" and "ulimit -d n", respectively.

'.' outside of string in pack
(F) The argument to a '.' in your template tried to move the working position to before the start of the packed string being built.
'@' outside of string in unpack
(F) You had a template that specified an absolute position outside the string being unpacked. See "pack" in perlfunc.
'@' outside of string with malformed UTF-8 in unpack
(F) You had a template that specified an absolute position outside the string being unpacked. The string being unpacked was also invalid UTF-8. See "pack" in perlfunc.
(W overload) The overload pragma was passed an argument it did not recognize. Did you mistype an operator?
(F) An object with an overloaded dereference operator was dereferenced, but the overloaded operation did not return a reference. See overload.
(F) An object with a "qr" overload was used as part of a match, but the overloaded operation didn't return a compiled regexp. See overload.
%s package attribute may clash with future reserved word: %s
(W reserved) A lowercase attribute name was used that had a package-specific handler. That name might have a meaning to Perl itself some day, even though it doesn't yet. Perhaps you should use a mixed-case attribute name, instead. See attributes.
(F) You can't specify a repeat count so large that it overflows your signed integers. See "pack" in perlfunc.
(W io) A single call to write() produced more lines than can fit on a page. See perlform.
(P) An internal error.
(P) One of the file test operators entered a code branch that calls an ACL related-function, but that function is not available on this platform. Earlier checks mean that it should not be possible to enter this branch on this platform.
(P) A child pseudo-process in the ithreads implementation on Windows was not scheduled within the time period allowed and therefore was not able to initialize properly.
(P) Failed an internal consistency check trying to compile a grep.
(P) The savestack was requested to restore more localized values than there are in the savestack.
(P) Failed an internal consistency check while trying to reset a weak reference.
(P) While attempting folding constants an exception other than an "eval" failure was caught.
(P) The library function frexp() failed, making printf("%f") impossible.
(P) We popped the context stack to a context with the specified label, and then discovered it wasn't a context we know how to do a goto in.
(P) The internal routine used to clear a typeglob's entries tried repeatedly, but each time something re-created entries in the glob. Most likely the glob contains an object with a reference back to the glob and a destructor that adds a new object to the glob.
(P) The lexer got into a bad state at a case modifier.
(P) The lexer got into a bad state parsing a string with brackets.
(F) A forked child returned an incomprehensible message about its errno.
(P) The savestack probably got out of sync. At least, there was an invalid enum on the top of it.
(P) Failed an internal consistency check while trying to reset all weak references to an object.
(P) Something requested a negative number of bytes of malloc.
(P) Something tried to allocate either more memory than possible or a negative amount.
(P) The parser failed an internal consistency check while trying to parse a "foreach" loop.
(P) The compiler got confused about which scratch pad it was allocating and freeing temporaries and lexicals from.
(P) The compiler got confused about which scratch pad it was allocating and freeing temporaries and lexicals from.
(P) A zero scratch pad offset was detected internally. An attempt was made to free a target that had not been allocated to begin with.
(P) The compiler got confused about which scratch pad it was allocating and freeing temporaries and lexicals from.
(P) A zero scratch pad offset was detected internally. Most likely an operator needed a target but that target had not been allocated for whatever reason.
(P) The compiler got confused about which scratch pad it was allocating and freeing temporaries and lexicals from.
(P) An invalid scratch pad offset was detected internally.
(P) The foreach iterator got called in a non-loop context frame.
(P) The internal pp_match() routine was called with invalid operational data.
(P) Something requested a negative number of bytes of realloc.
(P) The internal sv_replace() function was handed a new SV with a reference count other than 1.
(P) Some internal routine requested a goto (or something like it), and didn't supply the destination.
(P) We popped the context stack to a subroutine or eval context, and then discovered it wasn't a subroutine or eval context.
(P) scan_num() got called on something that wasn't a number.
(P) While compiling a pattern that has embedded (?{}) or (??{}) code blocks, perl couldn't locate the code block that should have already been seen and compiled by perl before control passed to the regex compiler.
(P) The sv_chop() routine was passed a position that is not within the scalar's string buffer.
(P) The sv_insert() routine was told to remove more string than there was string.
(P) The compiler attempted to do a goto, or something weird like that.
(P) When compiling a subroutine call in lvalue context, Perl failed an internal consistency check. It encountered a malformed op tree.
(P) The compiler is screwed up and attempted to use an op that isn't permitted at run time.
(P) The internal routine that handles arguments to &CORE::foo() subroutine calls was unable to determine what type of arguments were expected.
(P) Something tried to call utf16_to_utf8 with an odd (as opposed to even) byte length.
(P) Something tried to call utf16_to_utf8_reversed with an odd (as opposed to even) byte length.
(P) The lexer got into a bad state while processing a case modifier.
(W parenthesis) You said something like 

    my $foo, $bar = @_;

when you meant

    my ($foo, $bar) = @_;

Remember that "my", "our", "local" and "state" bind tighter than comma.

(F) Parsing code supplied by an extension violated the parser's API in a detectable way.
(F) You used a pattern that uses too many nested subpattern calls without consuming any text. Restructure the pattern so text is consumed before the nesting limit is exceeded.
"-p" destination: %s
(F) An error occurred during the implicit output invoked by the "-p" command-line switch. (This output goes to STDOUT unless you've redirected it with select().)
(F) The XS module in question was compiled against a different incompatible version of Perl than the one that has loaded the XS module.
(S regexp) You used a regular expression with case-insensitive matching, and there is a bug in Perl in which the built-in regular expression folding rules are not accurate. This may lead to incorrect results. Please report this as a bug to <https://github.com/Perl/perl5/issues/new/choose>.
(F) Your platform has very uncommon byte-order and integer size, so it was not possible to set up some or all fixed-width byte-order conversion functions. This is only a problem when you're using the '<' or '>' modifiers in (un)pack templates. See "pack" in perlfunc.
(F) The code you are trying to run has asked for a newer version of Perl than you are running. Perhaps "use 5.10" was written instead of "use 5.010" or "use v5.10". Without the leading "v", the number is interpreted as a decimal, with every three digits after the decimal point representing a part of the version number. So 5.10 is equivalent to v5.100.
(F) The module in question uses features of a version of Perl more recent than the currently running version. How long has it been since you upgraded, anyway? See "require" in perlfunc.
(F) An error peculiar to OS/2. PERL_SH_DIR is the directory to find the "sh"-shell in. See "PERL_SH_DIR" in perlos2.
(X) See "PERL_SIGNALS" in perlrun for legal values.
(F) The code you are trying to run claims it will not run on the version of Perl you are using because it is too new. Maybe the code needs to be updated, or maybe it is simply wrong and the version check should just be removed.
(S) PERL_HASH_SEED should match /^\s*(?:0x)?[0-9a-fA-F]+\s*\z/ but it contained a non hex character. This could mean you are not using the hash seed you think you are.
(S) The whole warning message will look something like: 

        perl: warning: Setting locale failed.
        perl: warning: Please check that your locale settings:
                LC_ALL = "En_US",
                LANG = (unset)
            are supported and installed on your system.
        perl: warning: Falling back to the standard locale ("C").

Exactly what were the failed locale settings varies. In the above the settings were that the LC_ALL was "En_US" and the LANG had no value. This error means that Perl detected that you and/or your operating system supplier and/or system administrator have set up the so-called locale system but Perl could not use those settings. This was not dead serious, fortunately: there is a "default locale" called "C" that Perl can and will use, and the script will be run. Before you really fix the problem, however, you will get the same error message each time you run Perl. How to really fix the problem can be found in perllocale section LOCALE PROBLEMS.

(S) Perl was run with the environment variable PERL_PERTURB_KEYS defined but containing an unexpected value. The legal values of this setting are as follows. 

  Numeric | String        | Result
  --------+---------------+-----------------------------------------
  0       | NO            | Disables key traversal randomization
  1       | RANDOM        | Enables full key traversal randomization
  2       | DETERMINISTIC | Enables repeatable key traversal
          |               | randomization

Both numeric and string values are accepted, but note that string values are case sensitive. The default for this setting is "RANDOM" or 1.

(W exec) A warning peculiar to VMS. Waitpid() was asked to wait for a process which isn't a subprocess of the current process. While this is fine from VMS' perspective, it's probably not what you intended.
'P' must have an explicit size in unpack
(F) The unpack format P must have an explicit size, not "*".
(F) The class in the character class [: :] syntax is unknown. The <-- HERE shows whereabouts in the regular expression the problem was discovered. Note that the POSIX character classes do not have the "is" prefix the corresponding C interfaces have: in other words, it's "[[:print:]]", not "isprint". See perlre.
(F) Your system has POSIX getpgrp(), which takes no argument, unlike the BSD version, which takes a pid.
(W regexp) Perl thinks that you intended to write a POSIX character class, but didn't use enough brackets. These POSIX class constructs [: :], [= =], and [. .] go inside character classes, the [] are part of the construct, for example: "qr/[012[:alpha:]345]/". What the regular expression pattern compiled to is probably not what you were intending. For example, "qr/[:alpha:]/" compiles to a regular bracketed character class consisting of the four characters ":", "a", "l", "h", and "p". To specify the POSIX class, it should have been written "qr/[[:alpha:]]/".

Note that [= =] and [. .] are not currently implemented; they are simply placeholders for future extensions and will cause fatal errors. The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.

If the specification of the class was not completely valid, the message indicates that.

(F) Within regular expression character classes ([]) the syntax beginning with "[." and ending with ".]" is reserved for future extensions. If you need to represent those character sequences inside a regular expression character class, just quote the square brackets with the backslash: "\[." and ".\]". The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.
(F) Within regular expression character classes ([]) the syntax beginning with "[=" and ending with "=]" is reserved for future extensions. If you need to represent those character sequences inside a regular expression character class, just quote the square brackets with the backslash: "\[=" and "=\]". The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.
(W qw) qw() lists contain items separated by whitespace; as with literal strings, comment characters are not ignored, but are instead treated as literal data. (You may have used different delimiters than the parentheses shown here; braces are also frequently used.)

You probably wrote something like this:

    @list = qw(
        a # a comment
        b # another comment
    );

when you should have written this:

    @list = qw(
        a
        b
    );

If you really want comments, build your list the old-fashioned way, with quotes and commas:

    @list = (
        'a',    # a comment
        'b',    # another comment
    );
(W qw) qw() lists contain items separated by whitespace; therefore commas aren't needed to separate the items. (You may have used different delimiters than the parentheses shown here; braces are also frequently used.)

You probably wrote something like this:

    qw! a, b, c !;

which puts literal commas into some of the list items. Write it without commas if you don't want them to appear in your data:

    qw! a b c !;
(F) An ioctl() or fcntl() returned more than Perl was bargaining for. Perl guesses a reasonable buffer size, but puts a sentinel byte at the end of the buffer just in case. This sentinel byte got clobbered, and Perl assumes that memory is now corrupted. See "ioctl" in perlfunc.
(W syntax) There is a possible problem with the mixing of a control flow operator (e.g. "return") and a low-precedence operator like "or". Consider: 

    sub { return $x or $y; }

This is parsed as:

    sub { (return $x) or $y; }

Which is effectively just:

    sub { return $x; }

Either use parentheses or the high-precedence variant of the operator.

Note this may be also triggered for constructs like:

    sub { 1 if die; }
(W precedence) Your program uses a bitwise logical operator in conjunction with a numeric comparison operator, like this : 

    if ($x & $y == 0) { ... }

This expression is actually equivalent to "$x & ($y == 0)", due to the higher precedence of "==". This is probably not what you want. (If you really meant to write this, disable the warning, or, better, put the parentheses explicitly and write "$x & ($y == 0)").

(W ambiguous) You said something like "m/$\/" in a regex. The regex "m/foo$\s+bar/m" translates to: match the word 'foo', the output record separator (see "$\" in perlvar) and the letter 's' (one time or more) followed by the word 'bar'.

If this is what you intended then you can silence the warning by using "m/${\}/" (for example: "m/foo${\}s+bar/").

If instead you intended to match the word 'foo' at the end of the line followed by whitespace and the word 'bar' on the next line then you can use "m/$(?)\/" (for example: "m/foo$(?)\s+bar/").

(W ambiguous) You said something like '@foo' in a double-quoted string but there was no array @foo in scope at the time. If you wanted a literal @foo, then write it as \@foo; otherwise find out what happened to the array you apparently lost track of.
(S precedence) The old irregular construct 

    open FOO || die;

is now misinterpreted as

    open(FOO || die);

because of the strict regularization of Perl 5's grammar into unary and list operators. (The old open was a little of both.) You must put parentheses around the filehandle, or use the new "or" operator instead of "||".

See "500 Server error".
(W closed) The filehandle you're writing to got itself closed sometime before now. Check your control flow.
(W closed) The filehandle you're printing on got itself closed sometime before now. Check your control flow.
(W) This is a standard message issued by OS/2 applications, while *nix applications die in silence. It is considered a feature of the OS/2 port. One can easily disable this by appropriate sighandlers, see "Signals" in perlipc. See also "Process terminated by SIGTERM/SIGINT" in perlos2.
(W illegalproto) A character follows % or @ in a prototype. This is useless, since % and @ gobble the rest of the subroutine arguments.
(S prototype) The subroutine being declared or defined had previously been declared or defined with a different function prototype.
(F) You've omitted the closing parenthesis in a function prototype definition.
(W prototype) A prototype was declared in both the parentheses after the sub name and via the prototype attribute. The prototype in parentheses is useless, since it will be replaced by the prototype from the attribute before it's ever used.
%s on BEGIN block ignored
(W syntax) "BEGIN" blocks are executed immediately after they are parsed and then thrown away. Any prototypes or attributes are therefore meaningless and are ignored. You should remove them from the "BEGIN" block. Note this also means you cannot create a constant called "BEGIN".
(F) You started a regular expression with a quantifier. Backslash it if you meant it literally. The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.
(F) There is currently a limit to the size of the min and max values of the {min,max} construct. The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.
(W regexp) Minima should be less than or equal to maxima. If you really want your regexp to match something 0 times, just put {0}.
(W regexp) You applied a regular expression quantifier in a place where it makes no sense, such as on a zero-width assertion. Try putting the quantifier inside the assertion instead. For example, the way to match "abc" provided that it is followed by three repetitions of "xyz" is "/abc(?=(?:xyz){3})/", not "/abc(?=xyz){3}/".
(F) One (or both) of the numeric arguments to the range operator ".." are outside the range which can be represented by integers internally. One possible workaround is to force Perl to use magical string increment by prepending "0" to your numbers.
(W regexp) (only under "use re 'strict'" or within "(?[...])")

Stricter rules help to find typos and other errors. Perhaps you didn't even intend a range here, if the "-" was meant to be some other character, or should have been escaped (like "\-"). If you did intend a range, the one that was used is not portable between ASCII and EBCDIC platforms, and doesn't have an obvious meaning to a casual reader.

 [3-7]    # OK; Obvious and portable
 [d-g]    # OK; Obvious and portable
 [A-Y]    # OK; Obvious and portable
 [A-z]    # WRONG; Not portable; not clear what is meant
 [a-Z]    # WRONG; Not portable; not clear what is meant
 [%-.]    # WRONG; Not portable; not clear what is meant
 [\x41-Z] # WRONG; Not portable; not obvious to non-geek

(You can force portability by specifying a Unicode range, which means that the endpoints are specified by "\N{...}", but the meaning may still not be obvious.) The stricter rules require that ranges that start or stop with an ASCII character that is not a control have all their endpoints be the literal character, and not some escape sequence (like "\x41"), and the ranges must be all digits, or all uppercase letters, or all lowercase letters.

(W regexp) (only under "use re 'strict'" or within "(?[...])")

Stricter rules help to find typos and other errors. You included a range, and at least one of the end points is a decimal digit. Under the stricter rules, when this happens, both end points should be digits in the same group of 10 consecutive digits.

(W io) The dirhandle you're reading from is either closed or not really a dirhandle. Check your control flow.
(W closed) The filehandle you're reading from got itself closed sometime before now. Check your control flow.
(W unopened) The filehandle you're reading from was never opened. Check your control flow.
(W closed) You tried to read from a closed filehandle.
(W unopened) You tried to read from a filehandle that was never opened.
(S malloc) An internal routine called realloc() on something that had already been freed.
(S debugging) You can't use the -D option unless the code to produce the desired output is compiled into Perl, which entails some overhead, which is why it's currently left out of your copy.
(P) It is currently not permitted to load modules when creating a filehandle inside an %INC hook. This can happen with "open my $fh, '<', \$scalar", which implicitly loads PerlIO::scalar. Try loading PerlIO::scalar explicitly first.
(F) While calculating the method resolution order (MRO) of a package, Perl believes it found an infinite loop in the @ISA hierarchy. This is a crude check that bails out after 100 levels of @ISA depth.
(W redundant) You called a function with more arguments than other arguments you supplied indicated would be needed. Currently only emitted when a printf-type format required fewer arguments than were supplied, but might be used in the future for e.g. "pack" in perlfunc.
(P) Perl's I/O implementation failed an internal consistency check. If you see this message, something is very wrong.
(W misc) You gave a single reference where Perl was expecting a list with an even number of elements (for assignment to a hash). This usually means that you used the anon hash constructor when you meant to use parens. In any case, a hash requires key/value pairs. 

    %hash = { one => 1, two => 2, };    # WRONG
    %hash = [ qw/ an anon array / ];    # WRONG
    %hash = ( one => 1, two => 2, );    # right
    %hash = qw( one 1 two 2 );                  # also fine
(W misc) You have attempted to weaken a reference that is already weak. Doing so has no effect.
(W misc) You have attempted to unweaken a reference that is not weak. Doing so has no effect.
(F) You used "\g0" or similar in a regular expression. You may refer to capturing parentheses only with strictly positive integers (normal backreferences) or with strictly negative integers (relative backreferences). Using 0 does not make sense.
(F) You used something like "\7" in your regular expression, but there are not at least seven sets of capturing parentheses in the expression. If you wanted to have the character with ordinal 7 inserted into the regular expression, prepend zeroes to make it three digits long: "\007"

The <-- HERE shows whereabouts in the regular expression the problem was discovered.

(F) You used something like "\k'NAME'" or "\k<NAME>" in your regular expression, but there is no corresponding named capturing parentheses such as "(?'NAME'...)" or "(?<NAME>...)". Check if the name has been spelled correctly both in the backreference and the declaration.

The <-- HERE shows whereabouts in the regular expression the problem was discovered.

(F) You used something like "\g{-7}" in your regular expression, but there are not at least seven sets of closed capturing parentheses in the expression before where the "\g{-7}" was located.

The <-- HERE shows whereabouts in the regular expression the problem was discovered.

(P) The regular expression engine got confused by what the regular expression compiler gave it.
(F) The regular expression pattern had too many occurrences of the specified modifier. Remove the extraneous ones.
(F) Turning off the given modifier has the side effect of turning on another one. Perl currently doesn't allow this. Reword the regular expression to use the modifier you want to turn on (and place it before the minus), instead of the one you want to turn off.
(F) The regular expression pattern had too many occurrences of the specified modifier. Remove the extraneous ones.
(F) The regular expression pattern had more than one of these mutually exclusive modifiers. Retain only the modifier that is supposed to be there.
(P) A "can't happen" error, because safemalloc() should have caught it earlier.
(F) Your format contains the ~~ repeat-until-blank sequence and a numeric field that will never go blank so that the repetition never terminates. You might use ^# instead. See perlform.
(W misc) You have used a replacement list that is longer than the search list. So the additional elements in the replacement list are meaningless.
(F) You called the constructor for a class that has a required named parameter, but did not pass that parameter at all.
'(*%s' requires a terminating ':' in regex; marked by <-- HERE in m/%s/
(F) You used a construct that needs a colon and pattern argument. Supply these or check that you are using the right construct.
'%s' resolved to '\o{%s}%d'
As of Perl 5.32, this message is no longer generated. Instead, see "Non-octal character '%c' terminates \o early. Resolved as "%s"". (W misc, regexp) You wrote something like "\08", or "\179" in a double-quotish string. All but the last digit is treated as a single character, specified in octal. The last digit is the next character in the string. To tell Perl that this is indeed what you want, you can use the "\o{ }" syntax, or use exactly three digits to specify the octal for the character.
(W syntax) You wrote your assignment operator backwards. The = must always come last, to avoid ambiguity with subsequent unary operators.
(W io) The dirhandle you tried to do a rewinddir() on is either closed or not really a dirhandle. Check your control flow.
(S internal) Something went wrong in Perl's internal bookkeeping of scalars: not all scalar variables were deallocated by the time Perl exited. What this usually indicates is a memory leak, which is of course bad, especially if the Perl program is intended to be long-running.
(W syntax) You've used an array slice (indicated by @) to select a single element of an array. Generally it's better to ask for a scalar value (indicated by $). The difference is that $foo[&bar] always behaves like a scalar, both when assigning to it and when evaluating its argument, while @foo[&bar] behaves like a list when you assign to it, and provides a list context to its subscript, which can do weird things if you're expecting only one subscript.

On the other hand, if you were actually hoping to treat the array element as a list, you need to look into how references work, because Perl will not magically convert between scalars and lists for you. See perlref.

(W syntax) You've used a hash slice (indicated by @) to select a single element of a hash. Generally it's better to ask for a scalar value (indicated by $). The difference is that $foo{&bar} always behaves like a scalar, both when assigning to it and when evaluating its argument, while @foo{&bar} behaves like a list when you assign to it, and provides a list context to its subscript, which can do weird things if you're expecting only one subscript.

On the other hand, if you were actually hoping to treat the hash element as a list, you need to look into how references work, because Perl will not magically convert between scalars and lists for you. See perlref.

(F) The lexer couldn't find the final delimiter of a // or m{} construct. Remember that bracketing delimiters count nesting level. Missing the leading "$" from a variable $m may cause this error.

Note that since Perl 5.10.0 a // can also be the defined-or construct, not just the empty search pattern. Therefore code written in Perl 5.10.0 or later that uses the // as the defined-or can be misparsed by pre-5.10.0 Perls as a non-terminated search pattern.

(W io) The dirhandle you are doing a seekdir() on is either closed or not really a dirhandle. Check your control flow.
%sseek() on unopened filehandle
(W unopened) You tried to use the seek() or sysseek() function on a filehandle that was either never opened or has since been closed.
(F) This machine doesn't implement the select() system call.
(F) Self-ties are of arrays and hashes are not supported in the current implementation.
(W semicolon) A nearby syntax error was probably caused by a missing semicolon, or possibly some other missing operator, such as a comma.
(S internal) The internal newSVsv() routine was called to duplicate a scalar that had previously been marked as free.
(F) You don't have System V semaphore IPC on your system.
(W closed) The socket you're sending to got itself closed sometime before now. Check your control flow.
(F) These three characters may not appear in sequence in a double-quotish context. This message is raised only on non-ASCII platforms (a different error message is output on ASCII ones). If you were intending to specify a control character with this sequence, you'll have to use a different way to specify it.
(F) A regular expression ended with an incomplete extension (?. The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.
(F) A proposed regular expression extension has the character reserved but has not yet been written. The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.
(F) You used a regular expression extension that doesn't make sense. The <-- HERE shows whereabouts in the regular expression the problem was discovered. This may happen when using the "(?^...)" construct to tell Perl to use the default regular expression modifiers, and you redundantly specify a default modifier. For other causes, see perlre.
(F) A regular expression comment must be terminated by a closing parenthesis. Embedded parentheses aren't allowed. See perlre.
(F) A named reference of the form "(?&...)" was missing the final closing parenthesis after the name. The <-- HERE shows whereabouts in the regular expression the problem was discovered.
(F) A named group of the form "(?'...')" or "(?<...>)" was missing the final closing quote or angle bracket. The <-- HERE shows whereabouts in the regular expression the problem was discovered.
(F) A lookahead assertion "(?=...)" or "(?!...)" or lookbehind assertion "(?<=...)" or "(?<!...)" was missing the final closing parenthesis. The <-- HERE shows whereabouts in the regular expression the problem was discovered.
(F) A named reference of the form "(?('...')...)" or "(?(<...>)...)" was missing the final closing quote or angle bracket after the name. The <-- HERE shows whereabouts in the regular expression the problem was discovered.
(F) There was no matching closing parenthesis for the '('. The <-- HERE shows whereabouts in the regular expression the problem was discovered.
(F) The regular expression expects a mandatory argument following the escape sequence and this has been omitted or incorrectly written.
(F) The end of the perl code contained within the {...} must be followed immediately by a ')'.
(F) A named reference of the form "(?P>...)" was missing the final closing parenthesis after the name. The <-- HERE shows whereabouts in the regular expression the problem was discovered.
(F) A named group of the form "(?P<...>')" was missing the final closing angle bracket. The <-- HERE shows whereabouts in the regular expression the problem was discovered.
(F) A named reference of the form "(?P=...)" was missing the final closing parenthesis after the name. The <-- HERE shows whereabouts in the regular expression the problem was discovered.
(F) An "(?R)" or "(?0)" sequence in a regular expression was missing the final parenthesis.
500 Server error
(A) This is the error message generally seen in a browser window when trying to run a CGI program (including SSI) over the web. The actual error text varies widely from server to server. The most frequently-seen variants are "500 Server error", "Method (something) not permitted", "Document contains no data", "Premature end of script headers", and "Did not produce a valid header".

This is a CGI error, not a Perl error.

You need to make sure your script is executable, is accessible by the user CGI is running the script under (which is probably not the user account you tested it under), does not rely on any environment variables (like PATH) from the user it isn't running under, and isn't in a location where the CGI server can't find it, basically, more or less. Please see the following for more information:

        https://www.perl.org/CGI_MetaFAQ.html
        http://www.htmlhelp.org/faq/cgifaq.html
        http://www.w3.org/Security/Faq/

You should also look at perlfaq9.

(F) You tried to assign to $), and your operating system doesn't support the setegid() system call (or equivalent), or at least Configure didn't think so.
(F) You tried to assign to $>, and your operating system doesn't support the seteuid() system call (or equivalent), or at least Configure didn't think so.
(F) Your system has the setpgrp() from BSD 4.2, which takes no arguments, unlike POSIX setpgid(), which takes a process ID and process group ID.
(F) You tried to assign to $(, and your operating system doesn't support the setrgid() system call (or equivalent), or at least Configure didn't think so.
(F) You tried to assign to $<, and your operating system doesn't support the setruid() system call (or equivalent), or at least Configure didn't think so.
(W closed) You tried to set a socket option on a closed socket. Did you forget to check the return value of your socket() call? See "setsockopt" in perlfunc.
(F) You assigned a reference to a scalar to $/ where the referenced item is not a positive integer. In older perls this appeared to work the same as setting it to "undef" but was in fact internally different, less efficient and with very bad luck could have resulted in your file being split by a stringified form of the reference.

In Perl 5.20.0 this was changed so that it would be exactly the same as setting $/ to undef, with the exception that this warning would be thrown.

You are recommended to change your code to set $/ to "undef" explicitly if you wish to slurp the file. As of Perl 5.28 assigning $/ to a reference to an integer which isn't positive is a fatal error.

(F) You tried to assign a reference to a non integer to $/. In older Perls this would have behaved similarly to setting it to a reference to a positive integer, where the integer was the address of the reference. As of Perl 5.20.0 this is a fatal error, to allow future versions of Perl to use non-integer refs for more interesting purposes.
(F) You don't have System V shared memory IPC on your system.
!=~ should be !~
(W syntax) The non-matching operator is !~, not !=~. !=~ will be interpreted as the != (numeric not equal) and ~ (1's complement) operators: probably not what you intended.
/%s/ should probably be written as "%s"
(W syntax) You have used a pattern where Perl expected to find a string, as in the first argument to "join". Perl will treat the true or false result of matching the pattern against $_ as the string, which is probably not what you had in mind.
(W closed) You tried to do a shutdown on a closed socket. Seems a bit superfluous.
(W signal) The signal handler named in %SIG doesn't, in fact, exist. Perhaps you put it into the wrong package?
(S) If you see this message, then something is seriously wrong with the internal bookkeeping of op trees. An op tree needed to be freed after a compilation error, but could not be found, so it was leaked instead.
(W overflow) You called "sleep" with a number that was larger than it can reliably handle and "sleep" probably slept for less time than requested.
(F) In a subroutine signature, you put something after a slurpy (array or hash) parameter. The slurpy parameter takes all the available arguments, so there can't be any left to fill later parameters.
(F) You should not use the "~~" operator on an object that does not overload it: Perl refuses to use the object's underlying structure for the smart match.
(D deprecated::smartmatch) This warning is emitted if you use the smartmatch ("~~") operator. This is a deprecated feature. Particularly, its behavior is noticed for being unnecessarily complex and unintuitive, and it will be removed in Perl 5.42.
(F) You tried to create a hash containing a very large key, where "very large" means that it needs at least 2 gigabytes to store. Unfortunately, Perl doesn't yet handle such large hash keys. You should reconsider your design to avoid hashing such a long string directly.
(F) An ancient error message that almost nobody ever runs into anymore. But before sort was a keyword, people sometimes used it as a filehandle.
(F) You tried to activate a source filter (usually by loading a source filter module) within a string passed to "eval". This is not permitted under the "unicode_eval" feature. Consider using "evalbytes" instead. See feature.
(W misc) You attempted to specify an offset that was past the end of the array passed to splice(). Splicing will instead commence at the end of the array, rather than past it. If this isn't what you want, try explicitly pre-extending the array by assigning $#array = $offset. See "splice" in perlfunc.
(P) The split was looping infinitely. (Obviously, a split shouldn't iterate more times than there are characters of input, which is what happened.) See "split" in perlfunc.
(W exec) You did an exec() with some statement after it other than a die(). This is almost always an error, because exec() never returns unless there was a failure. You probably wanted to use system() instead, which does return. To suppress this warning, put the exec() in a block by itself.
"state" subroutine %s can't be in a package
(F) Lexically scoped subroutines aren't in a package, so it doesn't make sense to try to declare one with a package qualifier on the front.
"state %s" used in sort comparison
(W syntax) The package variables $a and $b are used for sort comparisons. You used $a or $b in as an operand to the "<=>" or "cmp" operator inside a sort comparison block, and the variable had earlier been declared as a lexical variable. Either qualify the sort variable with the package name, or rename the lexical variable.
"state" variable %s can't be in a package
(F) Lexically scoped variables aren't in a package, so it doesn't make sense to try to declare one with a package qualifier on the front. Use local() if you want to localize a package variable.
(W unopened) You tried to use the stat() function on a filehandle that was either never opened or has since been closed.
(W utf8) You tried to open a reference to a scalar for read or append where the scalar contained code points over 0xFF. In-memory files model on-disk files and can only contain bytes.
(P) Overloading resolution over @ISA tree may be broken by importation stubs. Stubs should never be implicitly created, but explicit calls to "can" may break this.
(F) When subroutine signatures are enabled, any subroutine attributes must come before the signature. Note that this order was the opposite in versions 5.22..5.26. So: 

    sub foo :lvalue ($x, $y) { ... }  # 5.20 and 5.28 +
    sub foo ($x, $y) :lvalue { ... }  # 5.22 .. 5.26
(W closure) During compilation, an inner named subroutine or eval is attempting to capture an outer lexical subroutine that is not currently available. This can happen for one of two reasons. First, the lexical subroutine may be declared in an outer anonymous subroutine that has not yet been created. (Remember that named subs are created at compile time, while anonymous subs are created at run-time.) For example, 

    sub { my sub a {...} sub f { \&a } }

At the time that f is created, it can't capture the current "a" sub, since the anonymous subroutine hasn't been created yet. Conversely, the following won't give a warning since the anonymous subroutine has by now been created and is live:

    sub { my sub a {...} eval 'sub f { \&a }' }->();

The second situation is caused by an eval accessing a lexical subroutine that has gone out of scope, for example,

    sub f {
        my sub a {...}
        sub { eval '\&a' }
    }
    f()->();

Here, when the '\&a' in the eval is being compiled, f() is not currently being executed, so its &a is not available for capture.

"%s" subroutine &%s masks earlier declaration in same %s
(W shadow) A "my" or "state" subroutine has been redeclared in the current scope or statement, effectively eliminating all access to the previous instance. This is almost always a typographical error. Note that the earlier subroutine will still exist until the end of the scope or until all closure references to it are destroyed.
(W redefine) You redefined a subroutine. To suppress this warning, say 

    {
        no warnings 'redefine';
        eval "sub name { ... }";
    }
(W closure) An inner (nested) named subroutine is referencing a "my" subroutine defined in an outer named subroutine.

When the inner subroutine is called, it will see the value of the outer subroutine's lexical subroutine as it was before and during the *first* call to the outer subroutine; in this case, after the first call to the outer subroutine is complete, the inner and outer subroutines will no longer share a common value for the lexical subroutine. In other words, it will no longer be shared. This will especially make a difference if the lexical subroutines accesses lexical variables declared in its surrounding scope.

This problem can usually be solved by making the inner subroutine anonymous, using the "sub {}" syntax. When inner anonymous subs that reference lexical subroutines in outer subroutines are created, they are automatically rebound to the current values of such lexical subs.

(P) The substitution was looping infinitely. (Obviously, a substitution shouldn't iterate more times than there are characters of input, which is what happened.) See the discussion of substitution in "Regexp Quote-Like Operators" in perlop.
(F) The lexer couldn't find the interior delimiter of an s/// or s{}{} construct. Remember that bracketing delimiters count nesting level. Missing the leading "$" from variable $s may cause this error.
(F) The lexer couldn't find the final delimiter of an s/// or s{}{} construct. Remember that bracketing delimiters count nesting level. Missing the leading "$" from variable $s may cause this error.
(W substr)(F) You tried to reference a substr() that pointed outside of a string. That is, the absolute value of the offset was larger than the length of the string. See "substr" in perlfunc. This warning is fatal if substr is used in an lvalue context (as the left hand side of an assignment or as a subroutine argument for example).
(P) Perl tried to force the upgrade of an SV to a type which was actually inferior to its current type.
(F) A (?(condition)if-clause|else-clause) construct can have at most two branches (the if-clause and the else-clause). If you want one or both to contain alternation, such as using "this|that|other", enclose it in clustering parentheses: 

    (?(condition)(?:this|that|other)|else-clause)

The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.

(F) The condition part of a (?(condition)if-clause|else-clause) construct is not known. The condition must be one of the following: 

 (1) (2) ...        true if 1st, 2nd, etc., capture matched
 (<NAME>) ('NAME')  true if named capture matched
 (?=...) (?<=...)   true if subpattern matches
 (?!...) (?<!...)   true if subpattern fails to match
 (?{ CODE })        true if code returns a true value
 (R)                true if evaluating inside recursion
 (R1) (R2) ...      true if directly inside capture group 1, 2, etc.
 (R&NAME)           true if directly inside named capture
 (DEFINE)           always false; for defining named subpatterns

The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.

(F) You omitted to close a (?(condition)...) block somewhere in the pattern. Add a closing parenthesis in the appropriate position. See perlre.
(F) While under the "use filetest" pragma, we cannot switch the real and effective uids or gids.
(F) Probably means you had a syntax error. Common reasons include: 

    A keyword is misspelled.
    A semicolon is missing.
    A comma is missing.
    An opening or closing parenthesis is missing.
    An opening or closing brace is missing.
    A closing quote is missing.

Often there will be another error message associated with the syntax error giving more information. (Sometimes it helps to turn on -w.) The error message itself often tells you where it was in the line when it decided to give up. Sometimes the actual error is several tokens before this, because Perl is good at understanding random input. Occasionally the line number may be misleading, and once in a blue moon the only way to figure out what's triggering the error is to call "perl -c" repeatedly, chopping away half the program each time to see if the error went away. Sort of the cybernetic version of 20 questions.

(A) You've accidentally run your script through the Bourne shell instead of Perl. Check the #! line, or manually feed your script into Perl yourself.
(F) This error is likely to occur if you run a perl5 script through a perl4 interpreter, especially if the next 2 tokens are "use strict" or "my $var" or "our $var".
(F) Perl could not figure out what you meant inside this construct; this notifies you that it is giving up trying.
%s syntax OK
(F) The final summary message when a "perl -c" succeeds.
(W closed) You tried to read from a closed filehandle.
(W unopened) You tried to read from a filehandle that was never opened.
(F) You tried to do something with a function beginning with "sem", "shm", or "msg" but that System V IPC is not implemented in your machine. In some machines the functionality can exist but be unconfigured. Consult your system support.
(W closed) The filehandle you're writing to got itself closed sometime before now. Check your control flow.
"-T" and "-B" not implemented on filehandles
(F) Perl can't peek at the stdio buffer of filehandles when it doesn't know about your kind of stdio. You'll have to use a filename instead.
(F) You tried to use "goto" to reach a label that was too deeply nested for Perl to reach. Perl is doing you a favor by refusing.
(W io) The dirhandle you tried to telldir() is either closed or not really a dirhandle. Check your control flow.
(W unopened) You tried to use the tell() function on a filehandle that was either never opened or has since been closed.
(F) Configure couldn't find the crypt() function on your machine, probably because your vendor didn't supply it, probably because they think the U.S. Government thinks it's a secret, or at least that they will continue to pretend that it is. And if you quote me on that, I will deny it.
(F) To declare references to variables, as in "my \%x", you must first enable the feature: 

    no warnings "experimental::declared_refs";
    use feature "declared_refs";
(F) The function indicated isn't implemented on this architecture, according to the probings of Configure.
(S experimental::private_use) This feature is actually a hook for future use.
(F) It makes no sense to test the current stat buffer for symbolic linkhood if the last stat that wrote to the stat buffer already went past the symlink to get to the real file. Use an actual filename instead.
(S experimental::uniprop_wildcards) This feature is experimental and its behavior may in any future release of perl. See "Wildcards in Property Values" in perlunicode.
(F) This attribute was never supported on "my" or "sub" declarations.
(W internal) Warnings peculiar to VMS. You tried to change or delete an element of the CRTL's internal environ array, but your copy of Perl wasn't built with a CRTL that contained the setenv() function. You'll need to rebuild Perl with a CRTL that does, or redefine PERL_ENV_TABLES (see perlvms) so that the environ array isn't the target of the change to %ENV which produced the warning.
(F) Something has attempted to use an internal API call which depends on Perl being compiled with the default support for randomized hash key traversal, but this Perl has been compiled without it. You should report this warning to the relevant upstream party, or recompile perl with default options.
(F) You used a declaration similar to "my $x if 0". There has been a long-standing bug in Perl that causes a lexical variable not to be cleared at scope exit when its declaration includes a false conditional. Some people have exploited this bug to achieve a kind of static variable. Since we intend to fix this bug, we don't want people relying on this behavior. You can achieve a similar static effect by declaring the variable in a separate block outside the function, eg 

    sub f { my $x if 0; return $x++ }

becomes

    { my $x; sub f { return $x++ } }

Beginning with perl 5.10.0, you can also use "state" variables to have lexicals that are initialized only once (see feature):

    sub f { state $x; return $x++ }

This use of my() in a false conditional was deprecated beginning in Perl 5.10 and became a fatal error in Perl 5.30.

(F) The first time a user-defined property ("User-Defined Character Properties" in perlunicode) is used, its definition is looked up and converted into an internal form for more efficient handling in subsequent uses. There could be a race if two or more threads tried to do this processing nearly simultaneously. Instead, a critical section is created around this task, locking out all but one thread from doing it. This message indicates that the thread that is doing the conversion is taking an unexpectedly long time. The timeout exists solely to prevent deadlock; it's long enough that the system was likely thrashing and about to crash. There is no real remedy but rebooting.
(F) Your version of the C library apparently doesn't do times(). I suspect you're not running on Unix.
"-T" is on the #! line, it must also be used on the command line
(X) The #! line (or local equivalent) in a Perl script contains the -T option (or the -t option), but Perl was not invoked with -T in its command line. This is an error because, by the time Perl discovers a -T in a script, it's too late to properly taint everything from the environment. So Perl gives up.

If the Perl script is being executed as a command using the #! mechanism (or its local equivalent), this error can usually be fixed by editing the #! line so that the -%c option is a part of Perl's first argument: e.g. change "perl -n -%c" to "perl -%c -n".

If the Perl script is being executed as "perl scriptname", then the -%c option must appear on the command line: "perl -%c scriptname".

(F) You tried to define a customized To-mapping for lc(), lcfirst, uc(), or ucfirst() (or their string-inlined versions), but you specified an illegal mapping. See "User-Defined Character Properties" in perlunicode.
(F) Your template contains ()-groups with a ridiculously deep nesting level.
(F) There has to be at least one argument to syscall() to specify the system call to call, silly dilly.
(F) A subroutine using a signature fewer arguments than required by the signature. The caller of the subroutine is presumably at fault.

The message attempts to include the name of the called subroutine. If the subroutine has been aliased, the subroutine's original name will be shown, regardless of what name the caller used. It will also indicate the number of arguments given and the number expected.

Similar to the previous message but for subroutines that accept a variable number of arguments.
(X) The #! line (or local equivalent) in a Perl script contains the -M, -m or -C option.

In the case of -M and -m, this is an error because those options are not intended for use inside scripts. Use the "use" pragma instead.

The -C option only works if it is specified on the command line as well (with the same sequence of letters or numbers following). Either specify this option on the command line, or, if your system supports it, make your script executable and run it directly instead of passing it to perl.

(W void) A CHECK or INIT block is being defined during run time proper, when the opportunity to run them has already passed. Perhaps you are loading a file with "require" or "do" when you should be using "use" instead. Or perhaps you should put the "require" or "do" inside a BEGIN block.
(F) Perl supports a maximum of only 14 args to syscall().
(F) The function requires fewer arguments than you specified.
(F) A subroutine using a signature received more arguments than permitted by the signature. The caller of the subroutine is presumably at fault.

The message attempts to include the name of the called subroutine. If the subroutine has been aliased, the subroutine's original name will be shown, regardless of what name the caller used. It will also indicate the number of arguments given and the number expected.

Similar to the previous message but for subroutines that accept a variable number of arguments.
(F) You have exceeded the number of open "(" parentheses that haven't been matched by corresponding closing ones. This limit prevents eating up too much memory. It is initially set to 1000, but may be changed by setting "${^RE_COMPILE_RECURSION_LIMIT}" to some other value. This may need to be done in a BEGIN block before the regular expression pattern is compiled.
(F) You have executed code that nests too many BEGIN blocks inside of each other, either explicitly as BEGIN{} or implicitly as use statements. This limit defaults to a rather high number which should not be exceeded in normal circumstances, and triggering likely indicates something is very wrong in your code. For instance infinite recursion of eval and BEGIN blocks is known to trigger this error.

If you know that you have good reason to exceed the limit you can change it by setting "${^MAX_NESTED_EVAL_BEGIN_BLOCKS}" to a different value from the default of 1000.

(F) You have too many capture groups in your regex pattern. You need to rework your pattern to use less capture groups.
(A) You've accidentally run your script through csh instead of Perl. Check the #! line, or manually feed your script into Perl yourself.
(A) You've accidentally run your script through csh instead of Perl. Check the #! line, or manually feed your script into Perl yourself.
(F) The regular expression ends with an unbackslashed backslash. Backslash it. See perlre.
(F) The lexer couldn't find the interior delimiter of a tr/// or tr[][] or y/// or y[][] construct. Missing the leading "$" from variables $tr or $y may cause this error.
(F) The lexer couldn't find the final delimiter of a tr///, tr[][], y/// or y[][] construct.
(S) A package is using an old version of "Module::Install::DSL" to install, which makes unsafe assumptions about when INIT blocks will be called. Because "Module::Install::DSL" is used to install other modules and is difficult to upgrade we have a special workaround in place to deal with this. Unless you are a maintainer of an affected module you can ignore this warning. We emit it only as a sanity check.
'%s' trapped by operation mask
(F) You tried to use an operator from a Safe compartment in which it's disallowed. See Safe.
(F) Your machine doesn't implement a file truncation mechanism that Configure knows about.
(S experimental::try) This warning is emitted if you use the "try" and "catch" syntax with a "finally" block. This syntax is currently experimental and its behaviour may change in future releases of Perl.
(F) The subroutine in question in the CORE package requires its argument to be a hard reference to data of the specified type. Overloading is ignored, so a reference to an object that is not the specified type, but nonetheless has overloading to handle it, will still not be accepted.
(F) This function requires the argument in that position to be of a certain type. Arrays must be @NAME or "@{EXPR}". Hashes must be %NAME or "%{EXPR}". No implicit dereferencing is allowed--use the {EXPR} forms as an explicit dereference. See perlref.
(F) Your machine doesn't implement the umask function and you tried to use it to restrict permissions for yourself (EXPR & 0700).
(F) You asked UNIVERSAL to export something, but UNIVERSAL is the base class for all classes and contains no exportable symbols.
(S internal) The exit code detected an internal inconsistency in how many execution contexts were entered and left.
(S internal) The exit code detected an internal inconsistency in how many values were temporarily localized.
(S internal) The exit code detected an internal inconsistency in how many blocks were entered and left.
(S internal) On exit, Perl found some strings remaining in the shared string table used for copy on write and for hash keys. The entries should have been freed, so this indicates a bug somewhere.
(S internal) The exit code detected an internal inconsistency in how many mortal scalars were allocated and freed.
(F) The format indicated doesn't seem to exist. Perhaps it's really in another package? See perlform.
(F) The sort comparison routine specified doesn't seem to exist. Perhaps it's in a different package? See "sort" in perlfunc.
(F) The subroutine indicated hasn't been defined, or if it was, it has since been undefined.
(F) The anonymous subroutine you're trying to call hasn't been defined, or if it was, it has since been undefined.
(F) The sort comparison routine specified is declared but doesn't seem to have been defined yet. See "sort" in perlfunc.
(F) The format indicated doesn't seem to exist. Perhaps it's really in another package? See perlform.
(W misc) An undefined value was assigned to a typeglob, a la "*foo = undef". This does nothing. It's possible that you really mean "undef *foo".
%s: Undefined variable
(A) You've accidentally run your script through csh instead of Perl. Check the #! line, or manually feed your script into Perl yourself.
(F) The simple rule to remember, if you want to match a literal "{" character (U+007B "LEFT CURLY BRACKET") in a regular expression pattern, is to escape each literal instance of it in some way. Generally easiest is to precede it with a backslash, like "\{" or enclose it in square brackets ("[{]"). If the pattern delimiters are also braces, any matching right brace ("}") should also be escaped to avoid confusing the parser, for example, 

 qr{abc\{def\}ghi}

Forcing literal "{" characters to be escaped enables the Perl language to be extended in various ways in future releases. To avoid needlessly breaking existing code, the restriction is not enforced in contexts where there are unlikely to ever be extensions that could conflict with the use there of "{" as a literal. Those that are not potentially ambiguous do not warn; those that are do raise a non-deprecation warning.

The contexts where no warnings or errors are raised are:

  • as the first character in a pattern, or following "^" indicating to anchor the match to the beginning of a line.
  • as the first character following a "|" indicating alternation.
  • as the first character in a parenthesized grouping like 

     /foo({bar)/
 /foo(?:{bar)/
  • as the first character following a quantifier 

     /\s*{/
(W regexp) The simple rule to remember, if you want to match a literal "{" character (U+007B "LEFT CURLY BRACKET") in a regular expression pattern, is to escape each literal instance of it in some way. Generally easiest is to precede it with a backslash, like "\{" or enclose it in square brackets ("[{]"). If the pattern delimiters are also braces, any matching right brace ("}") should also be escaped to avoid confusing the parser, for example, 

 qr{abc\{def\}ghi}

Forcing literal "{" characters to be escaped enables the Perl language to be extended in various ways in future releases. To avoid needlessly breaking existing code, the restriction is not enforced in contexts where there are unlikely to ever be extensions that could conflict with the use there of "{" as a literal. Those that are not potentially ambiguous do not warn; those that are raise this warning. This makes sure that an inadvertent typo doesn't silently cause the pattern to compile to something unintended.

The contexts where no warnings or errors are raised are:

  • as the first character in a pattern, or following "^" indicating to anchor the match to the beginning of a line.
  • as the first character following a "|" indicating alternation.
  • as the first character in a parenthesized grouping like 

     /foo({bar)/
 /foo(?:{bar)/
  • as the first character following a quantifier 

     /\s*{/
(W regexp) (only under "use re 'strict'")

Within the scope of "use re 'strict'" in a regular expression pattern, you included an unescaped "}" or "]" which was interpreted literally. These two characters are sometimes metacharacters, and sometimes literals, depending on what precedes them in the pattern. This is unlike the similar ")" which is always a metacharacter unless escaped.

This action at a distance, perhaps a large distance, can lead to Perl silently misinterpreting what you meant, so when you specify that you want extra checking by "use re 'strict'", this warning is generated. If you meant the character as a literal, simply confirm that to Perl by preceding the character with a backslash, or make it into a bracketed character class (like "[}]"). If you meant it as closing a corresponding "[" or "{", you'll need to look back through the pattern to find out why that isn't happening.

(F) The unexec() routine failed for some reason. See your local FSF representative, who probably put it there in the first place.
(F) You had something like this: 

 (?[ | \p{Digit} ])

where the "|" is a binary operator with an operand on the right, but no operand on the left.

(F) You had something like this: 

 (?[ z ])

Within "(?[ ])", no literal characters are allowed unless they are within an inner pair of square brackets, like

 (?[ [ z ] ])

Another possibility is that you forgot a backslash. Perl isn't smart enough to figure out what you really meant.

(F) You tried to specify something other than a single class name with an optional trailing verison number as the value for a "class" ":isa" attribute. This confused the parser.
(S) exit() was called or the script otherwise finished gracefully when "PERL_EXIT_WARN" was set in "PL_exit_flags".
(S) An uncaught die() was called when "PERL_EXIT_WARN" was set in "PL_exit_flags".
(F) You had something like this: 

 (?[ ( \p{Digit} + ) ])

The ")" is out-of-place. Something apparently was supposed to be combined with the digits, or the "+" shouldn't be there, or something like that. Perl can't figure out what was intended.

(F) While parsing an extended character class a ']' character was encountered at a point in the definition where the only legal use of ']' is to close the character class definition as part of a '])', you may have forgotten the close paren, or otherwise confused the parser.
(F) You had something like this: 

 (?[ \p{Digit} ( \p{Lao} + \p{Thai} ) ])

There should be an operator before the "(", as there's no indication as to how the digits are to be combined with the characters in the Lao and Thai scripts.

(S nonchar) Certain codepoints, such as U+FFFE and U+FFFF, are defined by the Unicode standard to be non-characters. Those are legal codepoints, but are reserved for internal use; so, applications shouldn't attempt to exchange them. An application may not be expecting any of these characters at all, and receiving them may lead to bugs. If you know what you are doing you can turn off this warning by "no warnings 'nonchar';".

This is not really a "severe" error, but it is supposed to be raised by default even if warnings are not enabled, and currently the only way to do that in Perl is to mark it as serious.

(F) A Unicode property wildcard looks like a delimited regular expression pattern (all within the braces of the enclosing "\p{...}". The closing delimtter to match the opening one was not found. If the opening one is escaped by preceding it with a backslash, the closing one must also be so escaped.
(F) A Unicode string property is one which expands to a sequence of multiple characters. An example is "\p{name=KATAKANA LETTER AINU P}", which consists of the sequence "\N{KATAKANA LETTER SMALL H}" followed by "\N{COMBINING KATAKANA-HIRAGANA SEMI-VOICED SOUND MARK}". Extended character classes, "(?[...])" currently cannot handle these.
(S surrogate) You had a UTF-16 surrogate in a context where they are not considered acceptable. These code points, between U+D800 and U+DFFF (inclusive), are used by Unicode only for UTF-16. However, Perl internally allows all unsigned integer code points (up to the size limit available on your platform), including surrogates. But these can cause problems when being input or output, which is likely where this message came from. If you really really know what you are doing you can turn off this warning by "no warnings 'surrogate';".
(F) In Perl 5.12 and above, you have executed an ellipsis statement. This is a bare "...;", meant to be used to allow you to outline code that is to be written, but is not complete, similar to the following: 

    sub not_done_yet {
      my($self, $arg1, $arg2) = @_;
      ...
    }

If not_done_yet() is called, Perl will die with an "Unimplemented" error at the line containing "...".

(F) The name you used inside "\N{}" is unknown to Perl. Check the spelling. You can say "use charnames ":loose"" to not have to be so precise about spaces, hyphens, and capitalization on standard Unicode names. (Any custom aliases that have been created must be specified exactly, regardless of whether ":loose" is used or not.) This error may also happen if the "\N{}" is not in the scope of the corresponding "use charnames".
(F) The "(*" was followed by something that the regular expression compiler does not recognize. Check your spelling.
(P) Perl was about to print an error message in $@, but the $@ variable did not exist, even after an attempt to create it.
(W locale) You used a locale category that perl doesn't recognize, so it cannot carry out your request. Check that you are using a valid category. If so, see "Multi-threaded" in perllocale for advice on reporting this as a bug, and for modifying perl locally to accommodate your needs.
(F) The second argument of 3-argument open() is not among the list of valid modes: "<", ">", ">>", "+<", "+>", "+>>", "-|", "|-", "<&", ">&".
(W layer) An attempt was made to push an unknown layer onto the Perl I/O system. (Layers take care of transforming data between external and internal representations.) Note that some layers, such as "mmap", are not supported in all environments. If your program didn't explicitly request the failing operation, it may be the result of the value of the environment variable PERLIO.
(P) An error peculiar to VMS. Perl was reading values for %ENV before iterating over it, and someone else stuck a message in the stream of data Perl expected. Someone's very confused, or perhaps trying to subvert Perl's population of %ENV for nefarious purposes.
(F) Alphanumerics immediately following the closing delimiter of a regular expression pattern are interpreted by Perl as modifier flags for the regex. One of the ones you specified is invalid. One way this can happen is if you didn't put in white space between the end of the regex and a following alphanumeric operator: 

 if ($x =~ /foo/and $bar == 3) { ... }

The "a" is a valid modifier flag, but the "n" is not, and raises this error. Likely what was meant instead was:

 if ($x =~ /foo/ and $bar == 3) { ... }
(W) You tried to use an unknown subpragma of the "re" pragma.
(F) The condition part of a (?(condition)if-clause|else-clause) construct is not known. The condition must be one of the following: 

 (1) (2) ...            true if 1st, 2nd, etc., capture matched
 (<NAME>) ('NAME')      true if named capture matched
 (?=...) (?<=...)       true if subpattern matches
 (*pla:...) (*plb:...)  true if subpattern matches; also
                             (*positive_lookahead:...)
                             (*positive_lookbehind:...)
 (*nla:...) (*nlb:...)  true if subpattern fails to match; also
                             (*negative_lookahead:...)
                             (*negative_lookbehind:...)
 (?{ CODE })            true if code returns a true value
 (R)                    true if evaluating inside recursion
 (R1) (R2) ...          true if directly inside capture group 1, 2,
                             etc.
 (R&NAME)               true if directly inside named capture
 (DEFINE)               always false; for defining named subpatterns

The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.

(F) You specified an unknown Unicode option. See perlrun documentation of the "-C" switch for the list of known options.
(F) You specified an unknown Unicode option. See perlrun documentation of the "-C" switch for the list of known options.
(F) You specified to use a property within the "\p{...}" which was a syntactically valid user-defined property, but no definition was found for it by the time one was required to proceed. Check your spelling. See "User-Defined Character Properties" in perlunicode.
(F) You either made a typo or have incorrectly put a "*" quantifier after an open brace in your pattern. Check the pattern and review perlre for details on legal verb patterns.
(F) An error issued by the "warnings" pragma. You specified a warnings category that is unknown to perl at this point.

Note that if you want to enable a warnings category registered by a module (e.g. "use warnings 'File::Find'"), you must have loaded this module first.

(F) The brackets around a character class must match. If you wish to include a closing bracket in a character class, backslash it or put it first. The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.
(F) Unbackslashed parentheses must always be balanced in regular expressions. If you're a vi user, the % key is valuable for finding the matching parenthesis. The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.
(F) The lexer counted more closing curly or square brackets than opening ones, so you're probably missing a matching opening bracket. As a general rule, you'll find the missing one (so to speak) near the place you were last editing.
(W reserved) You used a bareword that might someday be claimed as a reserved word. It's best to put such a word in quotes, or capitalize it somehow, or insert an underbar into it. You might also declare it as a subroutine.
(F) The Perl parser has no idea what to do with the specified character in your Perl script (or eval) near the specified column. Perhaps you tried to run a compressed script, a binary program, or a directory as a Perl program.
(F) You attempted to add a named attribute to a "class" definition, but perl does not recognise the name of the requested attribute.
(F) You used a backslash-character combination which is not recognized by Perl inside character classes. This is a fatal error when the character class is used within "(?[ ])".
(W regexp) You used a backslash-character combination which is not recognized by Perl inside character classes. The character was understood literally, but this may change in a future version of Perl. The <-- HERE shows whereabouts in the regular expression the escape was discovered.
(W misc) You used a backslash-character combination which is not recognized by Perl. The character was understood literally, but this may change in a future version of Perl.
(W regexp) You used a backslash-character combination which is not recognized by Perl. The character(s) were understood literally, but this may change in a future version of Perl. The <-- HERE shows whereabouts in the regular expression the escape was discovered.
(F) You attempted to add a named attribute to a "field" definition, but perl does not recognise the name of the requested attribute.
(F) You called new on a class but supplied a parameter name that didn't match a class field name.
(F) You specified a signal name to the kill() function that was not recognized. Say "kill -l" in your shell to see the valid signal names on your system.
(F) You specified an illegal option to Perl. Don't do that. (If you think you didn't do that, check the #! line to see if it's supplying the bad switch on your behalf.)
(W newline) A file operation was attempted on a filename, and that operation failed, PROBABLY because the filename contained a newline, PROBABLY because you forgot to chomp() it off. See "chomp" in perlfunc.
(F) Your machine doesn't support opendir() and readdir().
(F) This machine doesn't implement the indicated function, apparently. At least, Configure doesn't think so.
(F) Your version of executable does not support forking.

Note that under some systems, like OS/2, there may be different flavors of Perl executables, some of which may support fork, some not. Try changing the name you call Perl by to "perl_", "perl__", and so on.

(F) Your program file begins with a Unicode Byte Order Mark (BOM) which declares it to be in a Unicode encoding that Perl cannot read.
(F) Your machine doesn't support the Berkeley socket mechanism, or at least that's what Configure thought.
(F) You used a pattern of the form "(*...:...)" but did not terminate the pattern with a ")". Fix the pattern and retry.
(F) The lexer found something other than a simple identifier at the start of an attribute, and it wasn't a semicolon or the start of a block. Perhaps you terminated the parameter list of the previous attribute too soon. See attributes.
(F) The lexer saw an opening (left) parenthesis character while parsing an attribute list, but the matching closing (right) parenthesis character was not found. You may need to add (or remove) a backslash character to get your parentheses to balance. See attributes.
(F) An argument to unpack("w",...) was incompatible with the BER compressed integer format and could not be converted to an integer. See "pack" in perlfunc.
(F) You used a pattern of the form "(*...)" but did not terminate the pattern with a ")". Fix the pattern and retry.
(F) This message occurs when a here document label has an initial quotation mark but the final quotation mark is missing. Perhaps you wrote: 

    <<"foo

instead of:

    <<"foo"
(F) In a regular expression, you had a "\g" that wasn't followed by a proper group reference. In the case of "\g{", the closing brace is missing; otherwise the "\g" must be followed by an integer. Fix the pattern and retry.
(F) The lexer saw a left angle bracket in a place where it was expecting a term, so it's looking for the corresponding right angle bracket, and not finding it. Chances are you left some needed parentheses out earlier in the line, and you really meant a "less than".
(F) You used a pattern of the form "(*VERB:ARG)" but did not terminate the pattern with a ")". Fix the pattern and retry.
(F) You used a pattern of the form "(*VERB)" but did not terminate the pattern with a ")". Fix the pattern and retry.
(W untie) A copy of the object returned from "tie" (or "tied") was still valid when "untie" was called.
(F) You called a POSIX function with incorrect arguments. See "FUNCTIONS" in POSIX for more information.
(F) You called a Win32 function with incorrect arguments. See Win32 for more information.
$[ used in %s (did you mean $] ?)
(W syntax) You used $[ in a comparison, such as: 

    if ($[ > 5.006) {
        ...
    }

You probably meant to use $] instead. $[ is the base for indexing arrays. $] is the Perl version number in decimal.

(F) The second listed construct is no longer legal. Use the first one instead.
(W misc) You assigned to an lvalue subroutine, but what the subroutine returned was a temporary scalar about to be discarded, so the assignment had no effect.
(W regexp) You have used an internal modifier such as (?-o) that has no meaning unless removed from the entire regexp: 

    if ($string =~ /(?-o)$pattern/o) { ... }

must be written as

    if ($string =~ /$pattern/) { ... }

The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.

(W syntax) The localization of lvalues such as local($x=10) is legal, but in fact the local() currently has no effect. This may change at some point in the future, but in the meantime such code is discouraged.
(W regexp) You have used an internal modifier such as (?o) that has no meaning unless applied to the entire regexp: 

    if ($string =~ /(?o)$pattern/) { ... }

must be written as

    if ($string =~ /$pattern/o) { ... }

The <-- HERE shows whereabouts in the regular expression the problem was discovered. See perlre.

(W misc) The "const" attribute has no effect except on anonymous closure prototypes. You applied it to a subroutine via attributes.pm. This is only useful inside an attribute handler for an anonymous subroutine.
(W misc) You have used the /d modifier where the searchlist has the same length as the replacelist. See perlop for more information about the /d modifier.
(W misc) You have a \E in a double-quotish string without a "\U", "\L" or "\Q" preceding it.
(W regexp) You specified something like these: 

 qr/a{3}?/
 qr/b{1,1}+/

The "?" and "+" don't have any effect, as they modify whether to match more or fewer when there is a choice, and by specifying to match exactly a given number, there is no room left for a choice.

(W scalar) You did something whose only interesting return value is a list without a side effect in scalar context, which does not accept a list.

For example

    my $x = sort @y;

This is not very useful, and perl currently optimizes this away.

(W void) You did something without a side effect in a context that does nothing with the return value, such as a statement that doesn't return a value from a block, or the left side of a scalar comma operator. Very often this points not to stupidity on your part, but a failure of Perl to parse your program the way you thought it would. For example, you'd get this if you mixed up your C precedence with Python precedence and said 

    $one, $two = 1, 2;

when you meant to say

    ($one, $two) = (1, 2);

Another common error is to use ordinary parentheses to construct a list reference when you should be using square or curly brackets, for example, if you say

    $array = (1,2);

when you should have said

    $array = [1,2];

The square brackets explicitly turn a list value into a scalar value, while parentheses do not. So when a parenthesized list is evaluated in a scalar context, the comma is treated like C's comma operator, which throws away the left argument, which is not what you want. See perlref for more on this.

This warning will not be issued for numerical constants equal to 0 or 1 since they are often used in statements like

    1 while sub_with_side_effects();

String constants that would normally evaluate to 0 or 1 are warned about.

(W regexp) The "p" modifier cannot be turned off once set. Trying to do so is futile.
(W) You did "use re;" without any arguments. That isn't very useful.
(W syntax) You used the push() or unshift() function with no arguments apart from the array, like push(@x) or unshift(@foo). That won't usually have any effect on the array, so is completely useless. It's possible in principle that push(@tied_array) could have some effect if the array is tied to a class which implements a PUSH method. If so, you can write it as "push(@tied_array,())" to avoid this warning.
"use" not allowed in expression
(F) The "use" keyword is recognized and executed at compile time, and returns no useful value. See perlmod.
(S experimental::args_array_with_signatures) An expression involving the @_ arguments array was found in a subroutine that uses a signature. This is experimental because the interaction between the arguments array and parameter handling via signatures is not guaranteed to remain stable in any future version of Perl, and such code should be avoided.
(F) You are now required to use the explicitly quoted form if you wish to use an empty line as the terminator of the here-document.

Use of a bare terminator was deprecated in Perl 5.000, and is a fatal error as of Perl 5.28.

(W regexp) You used the /c modifier in a substitution. The /c modifier is not presently meaningful in substitutions.
(W regexp) You used the /c modifier with a regex operand, but didn't use the /g modifier. Currently, /c is meaningful only when /g is used. (This may change in the future.)
(F) You used a code point that is not allowed, because it is too large. Unicode only allows code points up to 0x10FFFF, but Perl allows much larger ones. Earlier versions of Perl allowed code points above IV_MAX (0x7FFFFFF on 32-bit platforms, 0x7FFFFFFFFFFFFFFF on 64-bit platforms), however, this could possibly break the perl interpreter in some constructs, including causing it to hang in a few cases.

If your code is to run on various platforms, keep in mind that the upper limit depends on the platform. It is much larger on 64-bit word sizes than 32-bit ones.

The use of out of range code points was deprecated in Perl 5.24, and became a fatal error in Perl 5.28.

(S internal) The behavior of each() after insertion is undefined; it may skip items, or visit items more than once. Consider using keys() instead of each().
(F) The construction "my $x := 42" used to parse as equivalent to "my $x : = 42" (applying an empty attribute list to $x). This construct was deprecated in 5.12.0, and has now been made a syntax error, so ":=" can be reclaimed as a new operator in the future.

If you need an empty attribute list, for example in a code generator, add a space before the "=".

(W locale) You are matching a regular expression using locale rules, and the specified construct was encountered. This construct is only valid for UTF-8 locales, which the current locale isn't. This doesn't make sense. Perl will continue, assuming a Unicode (UTF-8) locale, but the results are likely to be wrong.
(F) Perhaps you modified the iterated array within the loop? This error is typically caused by code like the following: 

    @a = (3,4);
    @a = () for (1,2,@a);

You are not supposed to modify arrays while they are being iterated over. For speed and efficiency reasons, Perl internally does not do full reference-counting of iterated items, hence deleting such an item in the middle of an iteration causes Perl to see a freed value.

(W regexp) You used the /g modifier on the pattern for a "split" operator. Since "split" always tries to match the pattern repeatedly, the "/g" has no effect.
(D deprecated::goto_construct) Using "goto" to jump from an outer scope into an inner scope is deprecated and will be removed completely in Perl 5.42.

This was deprecated in Perl 5.12.

(D deprecated::unicode_property_name) Certain properties are deprecated by Unicode, and may eventually be removed from the Standard, at which time Perl will follow along. In the meantime, this message is raised to notify you.
(F) As an accidental feature, "AUTOLOAD" subroutines were looked up as methods (using the @ISA hierarchy), even when the subroutines to be autoloaded were called as plain functions (e.g. Foo::bar()), not as methods (e.g. "Foo->bar()" or "$obj->bar()").

This was deprecated in Perl 5.004, and was made fatal in Perl 5.28.

(F) You attempted to use a feature of printf that is accessible from only C. This usually means there's a better way to do it in Perl.
(D deprecated::delimiter_will_be_paired) You used the given character as a starting delimiter of a string outside the scope of "use feature 'extra_paired_delimiters'". This character is the mirror image of another Unicode character; within the scope of that feature, the two are considered a pair for delimitting strings. It is planned to make that feature the default, at which point this usage would become illegal; hence this warning.

For now, you may live with this warning, or turn it off, but this code will no longer compile in a future version of Perl. Or you can turn on "use feature 'extra_paired_delimiters'" and use the character that is the mirror image of this one for the closing string delimiter.

(S experimental::extra_paired_delimiters) This warning is emitted if you use as a string delimiter one of the non-ASCII mirror image ones enabled by "use feature 'extra_paired_delimiters'". Simply suppress the warning if you want to use the feature, but know that in doing so you are taking the risk of using an experimental feature which may change or be removed in a future Perl version:
(F) You were using a wildcard subpattern a Unicode property value, and the subpattern contained something that is illegal. Not all regular expression capabilities are legal in such subpatterns, and this is one. Rewrite your subppattern to not use the offending construct. See "Wildcards in Property Values" in perlunicode.
(W io) A filehandle represents an opened file, and when you opened the file it already went past any symlink you are presumably trying to look for. The operation returned "undef". Use a filename instead.
(W misc) You tried to use a reference as an array index; this probably isn't what you mean, because references in numerical context tend to be huge numbers, and so usually indicates programmer error.

If you really do mean it, explicitly numify your reference, like so: $array[0+$ref]. This warning is not given for overloaded objects, however, because you can overload the numification and stringification operators and then you presumably know what you are doing.

(F) You tried to use one of the string bitwise operators ("&" or "|" or "^" or "~") on a string containing a code point over 0xFF. The string bitwise operators treat their operands as strings of bytes, and values beyond 0xFF are nonsensical in this context.

Certain instances became fatal in Perl 5.28; others in perl 5.32.

(F) You tried to use "vec" on a string containing a code point over 0xFF, which is nonsensical here.

This became fatal in Perl 5.32.

(W taint, deprecated) You have supplied system() or exec() with multiple arguments and at least one of them is tainted. This used to be allowed but will become a fatal error in a future version of perl. Untaint your arguments. See perlsec.
(F) A grapheme is what appears to a native-speaker of a language to be a character. In Unicode (and hence Perl) a grapheme may actually be several adjacent characters that together form a complete grapheme. For example, there can be a base character, like "R" and an accent, like a circumflex "^", that appear when displayed to be a single character with the circumflex hovering over the "R". Perl currently allows things like that circumflex to be delimiters of strings, patterns, etc. When displayed, the circumflex would look like it belongs to the character just to the left of it. In order to move the language to be able to accept graphemes as delimiters, we cannot allow the use of delimiters which aren't graphemes by themselves. Also, a delimiter must already be assigned (or known to be never going to be assigned) to try to future-proof code, for otherwise code that works today would fail to compile if the currently unassigned delimiter ends up being something that isn't a stand-alone grapheme. Because Unicode is never going to assign non-character code points, nor code points that are above the legal Unicode maximum, those can be delimiters, and their use is legal.
(W uninitialized) An undefined value was used as if it were already defined. It was interpreted as a "" or a 0, but maybe it was a mistake. To suppress this warning assign a defined value to your variables.

To help you figure out what was undefined, perl will try to tell you the name of the variable (if any) that was undefined. In some cases it cannot do this, so it also tells you what operation you used the undefined value in. Note, however, that perl optimizes your program and the operation displayed in the warning may not necessarily appear literally in your program. For example, "that $foo" is usually optimized into ""that " . $foo", and the warning will refer to the "concatenation (.)" operator, even though there is no "." in your program.

"use re 'strict'" is experimental
(S experimental::re_strict) The things that are different when a regular expression pattern is compiled under 'strict' are subject to change in future Perl releases in incompatible ways. This means that a pattern that compiles today may not in a future Perl release. This warning is to alert you to that risk.
(F) Once you have a "use VERSION" declaration that calls for a Perl version of at least 5.39, you cannot have a subsequent one while the first is visible, even if it requests a later version.
(F) Once you have a "use VERSION" declaration in scope, you cannot have a subsequent one that calls for a Perl version of at least 5.39 while the first is visible.
(F) In a regular expression, you said something like 

 (?[ [ \xBEEF ] ])

Perl isn't sure if you meant this

 (?[ [ \x{BEEF} ] ])

or if you meant this

 (?[ [ \x{BE} E F ] ])

You need to add either braces or blanks to disambiguate.

(W regexp) Named Unicode character escapes "(\N{...})" may return a multi-character sequence. Even though a character class is supposed to match just one character of input, perl will match the whole thing correctly, except when the class is inverted ("[^...]"), or the escape is the beginning or final end point of a range. For these, what should happen isn't clear at all. In these circumstances, Perl discards all but the first character of the returned sequence, which is not likely what you want.
(W regexp) Extended character classes currently cannot handle operands that evaluate to more than one character. These are removed from the results of the expansion of the "\p{}".

This situation can happen, for example, in

 (?[ \p{name=/KATAKANA/} ])

"KATAKANA LETTER AINU P" is a legal Unicode name (technically a "named sequence"), but it is actually two characters. The above expression with match only the Unicode names containing KATAKANA that represent single characters.

(W regexp) You used a Unicode boundary ("\b{...}" or "\B{...}") in a portion of a regular expression where the character set modifiers "/a" or "/aa" are in effect. These two modifiers indicate an ASCII interpretation, and this doesn't make sense for a Unicode definition. The generated regular expression will compile so that the boundary uses all of Unicode. No other portion of the regular expression is affected.
(F) Using the "!~" operator with "s///r", "tr///r" or "y///r" is currently reserved for future use, as the exact behavior has not been decided. (Simply returning the boolean opposite of the modified string is usually not particularly useful.)
(S surrogate) You had a UTF-16 surrogate in a context where they are not considered acceptable. These code points, between U+D800 and U+DFFF (inclusive), are used by Unicode only for UTF-16. However, Perl internally allows all unsigned integer code points (up to the size limit available on your platform), including surrogates. But these can cause problems when being input or output, which is likely where this message came from. If you really really know what you are doing you can turn off this warning by "no warnings 'surrogate';".
(W misc) In a conditional expression, you used <HANDLE>, <*> (glob), each(), or readdir() as a boolean value. Each of these constructs can return a value of "0"; that would make the conditional expression false, which is probably not what you intended. When using these constructs in conditional expressions, test their values with the "defined" operator.
(W misc) A warning peculiar to VMS. Perl tried to read the value of an %ENV element from a CLI symbol table, and found a resultant string longer than 1024 characters. The return value has been truncated to 1024 characters.
(W closure) During compilation, an inner named subroutine or eval is attempting to capture an outer lexical that is not currently available. This can happen for one of two reasons. First, the outer lexical may be declared in an outer anonymous subroutine that has not yet been created. (Remember that named subs are created at compile time, while anonymous subs are created at run-time.) For example, 

    sub { my $x; sub f { $x } }

At the time that f is created, it can't capture the current value of $x, since the anonymous subroutine hasn't been created yet. Conversely, the following won't give a warning since the anonymous subroutine has by now been created and is live:

    sub { my $x; eval 'sub f { $x }' }->();

The second situation is caused by an eval accessing a variable that has gone out of scope, for example,

    sub f {
        my $x;
        sub { eval '$x' }
    }
    f()->();

Here, when the '$x' in the eval is being compiled, f() is not currently being executed, so its $x is not available for capture.

(S misc) With "use strict" in effect, you referred to a global variable that you apparently thought was imported from another module, because something else of the same name (usually a subroutine) is exported by that module. It usually means you put the wrong funny character on the front of your variable. It is also possible you used an "our" variable whose scope has ended.
(F) This message no longer should be raised as of Perl 5.30. It is retained in this document as a convenience for people using an earlier Perl version.

In Perl 5.30 and earlier, lookbehind is allowed only for subexpressions whose length is fixed and known at compile time. For positive lookbehind, you can use the "\K" regex construct as a way to get the equivalent functionality. See (?<=pattern) and \K in perlre.

Starting in Perl 5.18, there are non-obvious Unicode rules under "/i" that can match variably, but which you might not think could. For example, the substring "ss" can match the single character LATIN SMALL LETTER SHARP S. Here's a complete list of the current ones affecting ASCII characters:

   ASCII
  sequence      Matches single letter under /i
    FF          U+FB00 LATIN SMALL LIGATURE FF
    FFI         U+FB03 LATIN SMALL LIGATURE FFI
    FFL         U+FB04 LATIN SMALL LIGATURE FFL
    FI          U+FB01 LATIN SMALL LIGATURE FI
    FL          U+FB02 LATIN SMALL LIGATURE FL
    SS          U+00DF LATIN SMALL LETTER SHARP S
                U+1E9E LATIN CAPITAL LETTER SHARP S
    ST          U+FB06 LATIN SMALL LIGATURE ST
                U+FB05 LATIN SMALL LIGATURE LONG S T

This list is subject to change, but is quite unlikely to. Each ASCII sequence can be any combination of upper- and lowercase.

You can avoid this by using a bracketed character class in the lookbehind assertion, like

 (?<![sS]t)
 (?<![fF]f[iI])

This fools Perl into not matching the ligatures.

Another option for Perls starting with 5.16, if you only care about ASCII matches, is to add the "/aa" modifier to the regex. This will exclude all these non-obvious matches, thus getting rid of this message. You can also say

 use if $] ge 5.016, re => '/aa';

to apply "/aa" to all regular expressions compiled within its scope. See re.

(W) Variable length positive lookbehind with capturing is not well defined. This warning alerts you to the fact that you are using a construct which may change in a future version of perl. See the documentation of Positive Lookbehind in perlre for details. You may silence this warning with the following: 

    no warnings 'experimental::vlb';
(W) Variable length negative lookbehind with capturing is not well defined. This warning alerts you to the fact that you are using a construct which may change in a future version of perl. See the documentation of Negative Lookbehind in perlre for details. You may silence this warning with the following: 

    no warnings 'experimental::vlb';
"%s" variable %s masks earlier declaration in same %s
(W shadow) A "my", "our", "state" or "field" variable has been redeclared in the current scope or statement, effectively eliminating all access to the previous instance. This is almost always a typographical error. Note that the earlier variable will still exist until the end of the scope or until all closure references to it are destroyed.
(A) You've accidentally run your script through csh instead of Perl. Check the #! line, or manually feed your script into Perl yourself.
(W closure) An inner (nested) named subroutine is referencing a lexical variable defined in an outer named subroutine.

When the inner subroutine is called, it will see the value of the outer subroutine's variable as it was before and during the *first* call to the outer subroutine; in this case, after the first call to the outer subroutine is complete, the inner and outer subroutines will no longer share a common value for the variable. In other words, the variable will no longer be shared.

This problem can usually be solved by making the inner subroutine anonymous, using the "sub {}" syntax. When inner anonymous subs that reference variables in outer subroutines are created, they are automatically rebound to the current values of such variables.

(S printf) The %vd (s)printf format does not support version objects with alpha parts.
(F) You used a verb pattern that requires an argument. Supply an argument or check that you are using the right verb.
(F) You used a verb pattern that is not allowed an argument. Remove the argument or check that you are using the right verb.
(F) The parser found a line starting with "<<<<<<<", ">>>>>>>", or "=======". These may be left by a version control system to mark conflicts after a failed merge operation.
(P) The attempt to translate a "use Module n.n LIST" statement into its equivalent "BEGIN" block found an internal inconsistency with the version number.
(W misc) The version string contains invalid characters at the end, which are being ignored.
(W) You passed warn() an empty string (the equivalent of "warn """) or you called it with no args and $@ was empty.
(S) The implicit close() done by an open() got an error indication on the close(). This usually indicates your file system ran out of disk space.
(S io) There were errors during the implicit close() done on a filehandle when its reference count reached zero while it was still open, e.g.: 

    {
        open my $fh, '>', $file  or die "open: '$file': $!\n";
        print $fh $data or die "print: $!";
    } # implicit close here

Because various errors may only be detected by close() (e.g. buffering could allow the "print" in this example to return true even when the disk is full), it is dangerous to ignore its result. So when it happens implicitly, perl will signal errors by warning.

Prior to version 5.22.0, perl ignored such errors, so the common idiom shown above was liable to cause silent data loss.

(S ambiguous) You wrote a unary operator followed by something that looks like a binary operator that could also have been interpreted as a term or unary operator. For instance, if you know that the rand function has a default argument of 1.0, and you write 

    rand + 5;

you may THINK you wrote the same thing as

    rand() + 5;

but in actual fact, you got

    rand(+5);

So put in parentheses to say what you really mean.

(D deprecated::smartmatch) "when" depends on smartmatch, which is deprecated. Additionally, it has several special cases that may not be immediately obvious, and it will be removed in Perl 5.42. See the explanation under "Experimental Details on given and when" in perlsyn.
(S utf8) Perl met a wide character (ordinal >255) when it wasn't expecting one. This warning is by default on for I/O (like print).

If this warning does come from I/O, the easiest way to quiet it is simply to add the ":utf8" layer, e.g., "binmode STDOUT, ':utf8'". Another way to turn off the warning is to add "no warnings 'utf8';" but that is often closer to cheating. In general, you are supposed to explicitly mark the filehandle with an encoding, see open and "binmode" in perlfunc.

If the warning comes from other than I/O, this diagnostic probably indicates that incorrect results are being obtained. You should examine your code to determine how a wide character is getting to an operation that doesn't handle them.

(W locale) While in a single-byte locale (i.e., a non-UTF-8 one), a multi-byte character was encountered. Perl considers this character to be the specified Unicode code point. Combining non-UTF-8 locales and Unicode is dangerous. Almost certainly some characters will have two different representations. For example, in the ISO 8859-7 (Greek) locale, the code point 0xC3 represents a Capital Gamma. But so also does 0x393. This will make string comparisons unreliable.

You likely need to figure out how this multi-byte character got mixed up with your single-byte locale (or perhaps you thought you had a UTF-8 locale, but Perl disagrees).

(F) The count in the (un)pack template may be replaced by "[TEMPLATE]" only if "TEMPLATE" always matches the same amount of packed bytes that can be determined from the template alone. This is not possible if it contains any of the codes @, /, U, u, w or a *-length. Redesign the template.
(W syntax) It is possible that the @ISA contains a misspelled or never loaded package name, which can result in perl choosing an unexpected parent class's method to resolve the method call. If this is deliberate you can do something like 

  @Missing::Package::ISA = ();

to silence the warnings, otherwise you should correct the package name, or ensure that the package is loaded prior to the method call.

%s() with negative argument
(S misc) Certain operations make no sense with negative arguments. Warning is given and the operation is not done.
(W closed) The filehandle you're writing to got itself closed sometime before now. Check your control flow.
%s "\x%X" does not map to Unicode
(S utf8) When reading in different encodings, Perl tries to map everything into Unicode characters. The bytes you read in are not legal in this encoding. For example 

    utf8 "\xE4" does not map to Unicode

if you try to read in the a-diaereses Latin-1 as UTF-8.

'X' outside of string
(F) You had a (un)pack template that specified a relative position before the beginning of the string being (un)packed. See "pack" in perlfunc.
'x' outside of string in unpack
(F) You had an unpack template that specified a relative position after the end of the string being unpacked. See "pack" in perlfunc.
(F) And you probably never will, because you probably don't have the sources to your kernel, and your vendor probably doesn't give a rip about what you want. There is a vulnerability anywhere that you have a set-id script, and to close it you need to remove the set-id bit from the script that you're attempting to run. To actually run the script set-id, your best bet is to put a set-id C wrapper around your script.
(W syntax) You assigned a bareword as a signal handler name: 

    $SIG{...} = foo;

However, this will not make "sub foo" the signal handler. Instead, Perl will call the subroutine when the assignment is executed and use the returned value as the signal handler, which is probably not what you want. (If it is what you want, put "()" after the subroutine name to avoid the warning.)

To register the subroutine as the signal handler, take a reference to it:

    $SIG{...} = \&foo;
(F) When trying to initialize the random seed for hashes, Perl could not get any randomness out of your system. This usually indicates Something Very Wrong.
(F) Named Unicode character escapes ("\N{...}") may return a zero-length sequence. Such an escape was used in an extended character class, i.e. "(?[...])", or under "use re 'strict'", which is not permitted. Check that the correct escape has been used, and the correct charnames handler is in scope. The <-- HERE shows whereabouts in the regular expression the problem was discovered.

warnings, diagnostics.

2024-05-29 perl v5.40.0