3 perlvarcopy - Perl predefined variables
7 This is a pod file used for testing purposes by the test suite, please
12 =head2 Predefined Names
14 The following names have special meaning to Perl. Most
15 punctuation names have reasonable mnemonics, or analogs in the
16 shells. Nevertheless, if you wish to use long variable names,
21 at the top of your program. This aliases all the short names to the long
22 names in the current package. Some even have medium names, generally
23 borrowed from B<awk>. In general, it's best to use the
25 use English '-no_match_vars';
27 invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids
28 a certain performance hit with the use of regular expressions. See
31 Variables that depend on the currently selected filehandle may be set by
32 calling an appropriate object method on the IO::Handle object, although
33 this is less efficient than using the regular built-in variables. (Summary
34 lines below for this contain the word HANDLE.) First you must say
38 after which you may use either
46 Each method returns the old value of the IO::Handle attribute.
47 The methods each take an optional EXPR, which, if supplied, specifies the
48 new value for the IO::Handle attribute in question. If not supplied,
49 most methods do nothing to the current value--except for
50 autoflush(), which will assume a 1 for you, just to be different.
52 Because loading in the IO::Handle class is an expensive operation, you should
53 learn how to use the regular built-in variables.
55 A few of these variables are considered "read-only". This means that if
56 you try to assign to this variable, either directly or indirectly through
57 a reference, you'll raise a run-time exception.
59 You should be very careful when modifying the default values of most
60 special variables described in this document. In most cases you want
61 to localize these variables before changing them, since if you don't,
62 the change may affect other modules which rely on the default values
63 of the special variables that you have changed. This is one of the
64 correct ways to read the whole file at once:
66 open my $fh, "<", "foo" or die $!;
67 local $/; # enable localized slurp mode
71 But the following code is quite bad:
73 open my $fh, "<", "foo" or die $!;
74 undef $/; # enable slurp mode
78 since some other module, may want to read data from some file in the
79 default "line mode", so if the code we have just presented has been
80 executed, the global value of C<$/> is now changed for any other code
81 running inside the same Perl interpreter.
83 Usually when a variable is localized you want to make sure that this
84 change affects the shortest scope possible. So unless you are already
85 inside some short C<{}> block, you should create one yourself. For
89 open my $fh, "<", "foo" or die $!;
96 Here is an example of how your own code can go broken:
104 # do something with $_
107 You probably expect this code to print:
115 Why? Because nasty_break() modifies C<$_> without localizing it
116 first. The fix is to add local():
120 It's easy to notice the problem in such a short example, but in more
121 complicated code you are looking for trouble if you don't localize
122 changes to the special variables.
124 The following list is ordered by scalar variables first, then the
125 arrays, then the hashes.
134 The default input and pattern-searching space. The following pairs are
137 while (<>) {...} # equivalent only in while!
138 while (defined($_ = <>)) {...}
149 Here are the places where Perl will assume $_ even if you
156 The following functions:
158 abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, exp, glob,
159 hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print,
160 quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
161 rmdir, sin, split (on its second argument), sqrt, stat, study, uc, ucfirst,
166 All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
172 The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
173 when used without an C<=~> operator.
177 The default iterator variable in a C<foreach> loop if no other
178 variable is supplied.
182 The implicit iterator variable in the grep() and map() functions.
186 The implicit variable of given().
190 The default place to put an input record when a C<< <FH> >>
191 operation's result is tested by itself as the sole criterion of a C<while>
192 test. Outside a C<while> test, this will not happen.
196 As C<$_> is a global variable, this may lead in some cases to unwanted
197 side-effects. As of perl 5.9.1, you can now use a lexical version of
198 C<$_> by declaring it in a file or in a block with C<my>. Moreover,
199 declaring C<our $_> restores the global C<$_> in the current scope.
201 (Mnemonic: underline is understood in certain operations.)
212 Special package variables when using sort(), see L<perlfunc/sort>.
213 Because of this specialness $a and $b don't need to be declared
214 (using use vars, or our()) even when using the C<strict 'vars'> pragma.
215 Don't lexicalize them with C<my $a> or C<my $b> if you want to be
216 able to use them in the sort() comparison block or function.
225 Contains the subpattern from the corresponding set of capturing
226 parentheses from the last pattern match, not counting patterns
227 matched in nested blocks that have been exited already. (Mnemonic:
228 like \digits.) These variables are all read-only and dynamically
229 scoped to the current BLOCK.
236 The string matched by the last successful pattern match (not counting
237 any matches hidden within a BLOCK or eval() enclosed by the current
238 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
239 and dynamically scoped to the current BLOCK.
241 The use of this variable anywhere in a program imposes a considerable
242 performance penalty on all regular expression matches. See L</BUGS>.
244 See L</@-> for a replacement.
249 This is similar to C<$&> (C<$MATCH>) except that it does not incur the
250 performance penalty associated with that variable, and is only guaranteed
251 to return a defined value when the pattern was compiled or executed with
259 The string preceding whatever was matched by the last successful
260 pattern match (not counting any matches hidden within a BLOCK or eval
261 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
262 string.) This variable is read-only.
264 The use of this variable anywhere in a program imposes a considerable
265 performance penalty on all regular expression matches. See L</BUGS>.
267 See L</@-> for a replacement.
272 This is similar to C<$`> ($PREMATCH) except that it does not incur the
273 performance penalty associated with that variable, and is only guaranteed
274 to return a defined value when the pattern was compiled or executed with
282 The string following whatever was matched by the last successful
283 pattern match (not counting any matches hidden within a BLOCK or eval()
284 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
287 local $_ = 'abcdefghi';
289 print "$`:$&:$'\n"; # prints abc:def:ghi
291 This variable is read-only and dynamically scoped to the current BLOCK.
293 The use of this variable anywhere in a program imposes a considerable
294 performance penalty on all regular expression matches. See L</BUGS>.
296 See L</@-> for a replacement.
301 This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
302 performance penalty associated with that variable, and is only guaranteed
303 to return a defined value when the pattern was compiled or executed with
306 =item $LAST_PAREN_MATCH
309 X<$+> X<$LAST_PAREN_MATCH>
311 The text matched by the last bracket of the last successful search pattern.
312 This is useful if you don't know which one of a set of alternative patterns
313 matched. For example:
315 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
317 (Mnemonic: be positive and forward looking.)
318 This variable is read-only and dynamically scoped to the current BLOCK.
320 =item $LAST_SUBMATCH_RESULT
325 The text matched by the used group most-recently closed (i.e. the group
326 with the rightmost closing parenthesis) of the last successful search
327 pattern. (Mnemonic: the (possibly) Nested parenthesis that most
330 This is primarily used inside C<(?{...})> blocks for examining text
331 recently matched. For example, to effectively capture text to a variable
332 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
334 (?:(...)(?{ $var = $^N }))
336 By setting and then using C<$var> in this way relieves you from having to
337 worry about exactly which numbered set of parentheses they are.
339 This variable is dynamically scoped to the current BLOCK.
341 =item @LAST_MATCH_END
344 X<@+> X<@LAST_MATCH_END>
346 This array holds the offsets of the ends of the last successful
347 submatches in the currently active dynamic scope. C<$+[0]> is
348 the offset into the string of the end of the entire match. This
349 is the same value as what the C<pos> function returns when called
350 on the variable that was matched against. The I<n>th element
351 of this array holds the offset of the I<n>th submatch, so
352 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
353 past where $2 ends, and so on. You can use C<$#+> to determine
354 how many subgroups were in the last successful match. See the
355 examples given for the C<@-> variable.
357 =item %LAST_PAREN_MATCH
362 Similar to C<@+>, the C<%+> hash allows access to the named capture
363 buffers, should they exist, in the last successful match in the
364 currently active dynamic scope.
366 For example, C<$+{foo}> is equivalent to C<$1> after the following match:
368 'foo' =~ /(?<foo>foo)/;
370 The keys of the C<%+> hash list only the names of buffers that have
371 captured (and that are thus associated to defined values).
373 The underlying behaviour of C<%+> is provided by the
374 L<Tie::Hash::NamedCapture> module.
376 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
377 associated with the last successful regular expression. Therefore mixing
378 iterative access to them via C<each> may have unpredictable results.
379 Likewise, if the last successful match changes, then the results may be
382 =item HANDLE->input_line_number(EXPR)
384 =item $INPUT_LINE_NUMBER
389 X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
391 Current line number for the last filehandle accessed.
393 Each filehandle in Perl counts the number of lines that have been read
394 from it. (Depending on the value of C<$/>, Perl's idea of what
395 constitutes a line may not match yours.) When a line is read from a
396 filehandle (via readline() or C<< <> >>), or when tell() or seek() is
397 called on it, C<$.> becomes an alias to the line counter for that
400 You can adjust the counter by assigning to C<$.>, but this will not
401 actually move the seek pointer. I<Localizing C<$.> will not localize
402 the filehandle's line count>. Instead, it will localize perl's notion
403 of which filehandle C<$.> is currently aliased to.
405 C<$.> is reset when the filehandle is closed, but B<not> when an open
406 filehandle is reopened without an intervening close(). For more
407 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
408 an explicit close, line numbers increase across ARGV files (but see
409 examples in L<perlfunc/eof>).
411 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
412 line counter for a given filehandle without having to worry about
413 which handle you last accessed.
415 (Mnemonic: many programs use "." to mean the current line number.)
417 =item IO::Handle->input_record_separator(EXPR)
419 =item $INPUT_RECORD_SEPARATOR
424 X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
426 The input record separator, newline by default. This
427 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
428 variable, including treating empty lines as a terminator if set to
429 the null string. (An empty line cannot contain any spaces
430 or tabs.) You may set it to a multi-character string to match a
431 multi-character terminator, or to C<undef> to read through the end
432 of file. Setting it to C<"\n\n"> means something slightly
433 different than setting to C<"">, if the file contains consecutive
434 empty lines. Setting to C<""> will treat two or more consecutive
435 empty lines as a single empty line. Setting to C<"\n\n"> will
436 blindly assume that the next input character belongs to the next
437 paragraph, even if it's a newline. (Mnemonic: / delimits
438 line boundaries when quoting poetry.)
440 local $/; # enable "slurp" mode
441 local $_ = <FH>; # whole file now here
444 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
445 better for something. :-)
447 Setting C<$/> to a reference to an integer, scalar containing an integer, or
448 scalar that's convertible to an integer will attempt to read records
449 instead of lines, with the maximum record size being the referenced
452 local $/ = \32768; # or \"32768", or \$var_containing_32768
453 open my $fh, "<", $myfile or die $!;
456 will read a record of no more than 32768 bytes from FILE. If you're
457 not reading from a record-oriented file (or your OS doesn't have
458 record-oriented files), then you'll likely get a full chunk of data
459 with every read. If a record is larger than the record size you've
460 set, you'll get the record back in pieces. Trying to set the record
461 size to zero or less will cause reading in the (rest of the) whole file.
463 On VMS, record reads are done with the equivalent of C<sysread>,
464 so it's best not to mix record and non-record reads on the same
465 file. (This is unlikely to be a problem, because any file you'd
466 want to read in record mode is probably unusable in line mode.)
467 Non-VMS systems do normal I/O, so it's safe to mix record and
468 non-record reads of a file.
470 See also L<perlport/"Newlines">. Also see C<$.>.
472 =item HANDLE->autoflush(EXPR)
474 =item $OUTPUT_AUTOFLUSH
477 X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
479 If set to nonzero, forces a flush right away and after every write
480 or print on the currently selected output channel. Default is 0
481 (regardless of whether the channel is really buffered by the
482 system or not; C<$|> tells you only whether you've asked Perl
483 explicitly to flush after each write). STDOUT will
484 typically be line buffered if output is to the terminal and block
485 buffered otherwise. Setting this variable is useful primarily when
486 you are outputting to a pipe or socket, such as when you are running
487 a Perl program under B<rsh> and want to see the output as it's
488 happening. This has no effect on input buffering. See L<perlfunc/getc>
489 for that. See L<perlfunc/select> on how to select the output channel.
490 See also L<IO::Handle>. (Mnemonic: when you want your pipes to be piping hot.)
492 =item IO::Handle->output_field_separator EXPR
494 =item $OUTPUT_FIELD_SEPARATOR
499 X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
501 The output field separator for the print operator. If defined, this
502 value is printed between each of print's arguments. Default is C<undef>.
503 (Mnemonic: what is printed when there is a "," in your print statement.)
505 =item IO::Handle->output_record_separator EXPR
507 =item $OUTPUT_RECORD_SEPARATOR
512 X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
514 The output record separator for the print operator. If defined, this
515 value is printed after the last of print's arguments. Default is C<undef>.
516 (Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
517 Also, it's just like C<$/>, but it's what you get "back" from Perl.)
519 =item $LIST_SEPARATOR
522 X<$"> X<$LIST_SEPARATOR>
524 This is like C<$,> except that it applies to array and slice values
525 interpolated into a double-quoted string (or similar interpreted
526 string). Default is a space. (Mnemonic: obvious, I think.)
528 =item $SUBSCRIPT_SEPARATOR
533 X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
535 The subscript separator for multidimensional array emulation. If you
536 refer to a hash element as
542 $foo{join($;, $a, $b, $c)}
546 @foo{$a,$b,$c} # a slice--note the @
550 ($foo{$a},$foo{$b},$foo{$c})
552 Default is "\034", the same as SUBSEP in B<awk>. If your
553 keys contain binary data there might not be any safe value for C<$;>.
554 (Mnemonic: comma (the syntactic subscript separator) is a
555 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
556 taken for something more important.)
558 Consider using "real" multidimensional arrays as described
561 =item HANDLE->format_page_number(EXPR)
563 =item $FORMAT_PAGE_NUMBER
566 X<$%> X<$FORMAT_PAGE_NUMBER>
568 The current page number of the currently selected output channel.
570 (Mnemonic: % is page number in B<nroff>.)
572 =item HANDLE->format_lines_per_page(EXPR)
574 =item $FORMAT_LINES_PER_PAGE
577 X<$=> X<$FORMAT_LINES_PER_PAGE>
579 The current page length (printable lines) of the currently selected
580 output channel. Default is 60.
582 (Mnemonic: = has horizontal lines.)
584 =item HANDLE->format_lines_left(EXPR)
586 =item $FORMAT_LINES_LEFT
589 X<$-> X<$FORMAT_LINES_LEFT>
591 The number of lines left on the page of the currently selected output
594 (Mnemonic: lines_on_page - lines_printed.)
596 =item @LAST_MATCH_START
599 X<@-> X<@LAST_MATCH_START>
601 $-[0] is the offset of the start of the last successful match.
602 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
603 I<n>-th subpattern, or undef if the subpattern did not match.
605 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
606 $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
607 $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
608 C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
609 matched subgroup in the last successful match. Contrast with
610 C<$#+>, the number of subgroups in the regular expression. Compare
613 This array holds the offsets of the beginnings of the last
614 successful submatches in the currently active dynamic scope.
615 C<$-[0]> is the offset into the string of the beginning of the
616 entire match. The I<n>th element of this array holds the offset
617 of the I<n>th submatch, so C<$-[1]> is the offset where $1
618 begins, C<$-[2]> the offset where $2 begins, and so on.
620 After a match against some variable $var:
624 =item C<$`> is the same as C<substr($var, 0, $-[0])>
626 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
628 =item C<$'> is the same as C<substr($var, $+[0])>
630 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
632 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
634 =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
641 Similar to C<%+>, this variable allows access to the named capture buffers
642 in the last successful match in the currently active dynamic scope. To
643 each capture buffer name found in the regular expression, it associates a
644 reference to an array containing the list of values captured by all
645 buffers with that name (should there be several of them), in the order
650 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
651 foreach my $bufname (sort keys %-) {
652 my $ary = $-{$bufname};
653 foreach my $idx (0..$#$ary) {
654 print "\$-{$bufname}[$idx] : ",
655 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
668 The keys of the C<%-> hash correspond to all buffer names found in
669 the regular expression.
671 The behaviour of C<%-> is implemented via the
672 L<Tie::Hash::NamedCapture> module.
674 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
675 associated with the last successful regular expression. Therefore mixing
676 iterative access to them via C<each> may have unpredictable results.
677 Likewise, if the last successful match changes, then the results may be
680 =item HANDLE->format_name(EXPR)
685 X<$~> X<$FORMAT_NAME>
687 The name of the current report format for the currently selected output
688 channel. Default is the name of the filehandle. (Mnemonic: brother to
691 =item HANDLE->format_top_name(EXPR)
693 =item $FORMAT_TOP_NAME
696 X<$^> X<$FORMAT_TOP_NAME>
698 The name of the current top-of-page format for the currently selected
699 output channel. Default is the name of the filehandle with _TOP
700 appended. (Mnemonic: points to top of page.)
702 =item IO::Handle->format_line_break_characters EXPR
704 =item $FORMAT_LINE_BREAK_CHARACTERS
707 X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
709 The current set of characters after which a string may be broken to
710 fill continuation fields (starting with ^) in a format. Default is
711 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
712 poetry is a part of a line.)
714 =item IO::Handle->format_formfeed EXPR
716 =item $FORMAT_FORMFEED
719 X<$^L> X<$FORMAT_FORMFEED>
721 What formats output as a form feed. Default is \f.
726 X<$^A> X<$ACCUMULATOR>
728 The current value of the write() accumulator for format() lines. A format
729 contains formline() calls that put their result into C<$^A>. After
730 calling its format, write() prints out the contents of C<$^A> and empties.
731 So you never really see the contents of C<$^A> unless you call
732 formline() yourself and then look at it. See L<perlform> and
733 L<formline|perlfunc/formline PICTURE,LIST>.
738 X<$?> X<$CHILD_ERROR>
740 The status returned by the last pipe close, backtick (C<``>) command,
741 successful call to wait() or waitpid(), or from the system()
742 operator. This is just the 16-bit status word returned by the
743 traditional Unix wait() system call (or else is made up to look like it). Thus, the
744 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
745 C<$? & 127> gives which signal, if any, the process died from, and
746 C<$? & 128> reports whether there was a core dump. (Mnemonic:
747 similar to B<sh> and B<ksh>.)
749 Additionally, if the C<h_errno> variable is supported in C, its value
750 is returned via $? if any C<gethost*()> function fails.
752 If you have installed a signal handler for C<SIGCHLD>, the
753 value of C<$?> will usually be wrong outside that handler.
755 Inside an C<END> subroutine C<$?> contains the value that is going to be
756 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
757 change the exit status of your program. For example:
760 $? = 1 if $? == 255; # die would make it 255
763 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
764 actual VMS exit status, instead of the default emulation of POSIX
765 status; see L<perlvms/$?> for details.
767 Also see L<Error Indicators>.
769 =item ${^CHILD_ERROR_NATIVE}
770 X<$^CHILD_ERROR_NATIVE>
772 The native status returned by the last pipe close, backtick (C<``>)
773 command, successful call to wait() or waitpid(), or from the system()
774 operator. On POSIX-like systems this value can be decoded with the
775 WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, WSTOPSIG
776 and WIFCONTINUED functions provided by the L<POSIX> module.
778 Under VMS this reflects the actual VMS exit status; i.e. it is the same
779 as $? when the pragma C<use vmsish 'status'> is in effect.
784 The I<object reference> to the Encode object that is used to convert
785 the source code to Unicode. Thanks to this variable your perl script
786 does not have to be written in UTF-8. Default is I<undef>. The direct
787 manipulation of this variable is highly discouraged.
794 X<$!> X<$ERRNO> X<$OS_ERROR>
796 If used numerically, yields the current value of the C C<errno>
797 variable, or in other words, if a system or library call fails, it
798 sets this variable. This means that the value of C<$!> is meaningful
799 only I<immediately> after a B<failure>:
801 if (open my $fh, "<", $filename) {
802 # Here $! is meaningless.
805 # ONLY here is $! meaningful.
807 # Already here $! might be meaningless.
809 # Since here we might have either success or failure,
810 # here $! is meaningless.
812 In the above I<meaningless> stands for anything: zero, non-zero,
813 C<undef>. A successful system or library call does B<not> set
814 the variable to zero.
816 If used as a string, yields the corresponding system error string.
817 You can assign a number to C<$!> to set I<errno> if, for instance,
818 you want C<"$!"> to return the string for error I<n>, or you want
819 to set the exit value for the die() operator. (Mnemonic: What just
822 Also see L<Error Indicators>.
831 Each element of C<%!> has a true value only if C<$!> is set to that
832 value. For example, C<$!{ENOENT}> is true if and only if the current
833 value of C<$!> is C<ENOENT>; that is, if the most recent error was
834 "No such file or directory" (or its moral equivalent: not all operating
835 systems give that exact error, and certainly not all languages).
836 To check if a particular key is meaningful on your system, use
837 C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
838 See L<Errno> for more information, and also see above for the
841 =item $EXTENDED_OS_ERROR
844 X<$^E> X<$EXTENDED_OS_ERROR>
846 Error information specific to the current operating system. At
847 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
848 (and for MacPerl). On all other platforms, C<$^E> is always just
851 Under VMS, C<$^E> provides the VMS status value from the last
852 system error. This is more specific information about the last
853 system error than that provided by C<$!>. This is particularly
854 important when C<$!> is set to B<EVMSERR>.
856 Under OS/2, C<$^E> is set to the error code of the last call to
857 OS/2 API either via CRT, or directly from perl.
859 Under Win32, C<$^E> always returns the last error information
860 reported by the Win32 call C<GetLastError()> which describes
861 the last error from within the Win32 API. Most Win32-specific
862 code will report errors via C<$^E>. ANSI C and Unix-like calls
863 set C<errno> and so most portable Perl code will report errors
866 Caveats mentioned in the description of C<$!> generally apply to
867 C<$^E>, also. (Mnemonic: Extra error explanation.)
869 Also see L<Error Indicators>.
876 The Perl syntax error message from the last eval() operator.
877 If $@ is the null string, the last eval() parsed and executed
878 correctly (although the operations you invoked may have failed in the
879 normal fashion). (Mnemonic: Where was the syntax error "at"?)
881 Warning messages are not collected in this variable. You can,
882 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
885 Also see L<Error Indicators>.
892 X<$$> X<$PID> X<$PROCESS_ID>
894 The process number of the Perl running this script. You should
895 consider this variable read-only, although it will be altered
896 across fork() calls. (Mnemonic: same as shells.)
898 Note for Linux users: on Linux, the C functions C<getpid()> and
899 C<getppid()> return different values from different threads. In order to
900 be portable, this behavior is not reflected by C<$$>, whose value remains
901 consistent across threads. If you want to call the underlying C<getpid()>,
902 you may use the CPAN module C<Linux::Pid>.
909 X<< $< >> X<$UID> X<$REAL_USER_ID>
911 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
912 if you're running setuid.) You can change both the real uid and
913 the effective uid at the same time by using POSIX::setuid(). Since
914 changes to $< require a system call, check $! after a change attempt to
915 detect any possible errors.
917 =item $EFFECTIVE_USER_ID
922 X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
924 The effective uid of this process. Example:
926 $< = $>; # set real to effective uid
927 ($<,$>) = ($>,$<); # swap real and effective uid
929 You can change both the effective uid and the real uid at the same
930 time by using POSIX::setuid(). Changes to $> require a check to $!
931 to detect any possible errors after an attempted change.
933 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
934 C<< $< >> and C<< $> >> can be swapped only on machines
935 supporting setreuid().
942 X<$(> X<$GID> X<$REAL_GROUP_ID>
944 The real gid of this process. If you are on a machine that supports
945 membership in multiple groups simultaneously, gives a space separated
946 list of groups you are in. The first number is the one returned by
947 getgid(), and the subsequent ones by getgroups(), one of which may be
948 the same as the first number.
950 However, a value assigned to C<$(> must be a single number used to
951 set the real gid. So the value given by C<$(> should I<not> be assigned
952 back to C<$(> without being forced numeric, such as by adding zero. Note
953 that this is different to the effective gid (C<$)>) which does take a
956 You can change both the real gid and the effective gid at the same
957 time by using POSIX::setgid(). Changes to $( require a check to $!
958 to detect any possible errors after an attempted change.
960 (Mnemonic: parentheses are used to I<group> things. The real gid is the
961 group you I<left>, if you're running setgid.)
963 =item $EFFECTIVE_GROUP_ID
968 X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
970 The effective gid of this process. If you are on a machine that
971 supports membership in multiple groups simultaneously, gives a space
972 separated list of groups you are in. The first number is the one
973 returned by getegid(), and the subsequent ones by getgroups(), one of
974 which may be the same as the first number.
976 Similarly, a value assigned to C<$)> must also be a space-separated
977 list of numbers. The first number sets the effective gid, and
978 the rest (if any) are passed to setgroups(). To get the effect of an
979 empty list for setgroups(), just repeat the new effective gid; that is,
980 to force an effective gid of 5 and an effectively empty setgroups()
981 list, say C< $) = "5 5" >.
983 You can change both the effective gid and the real gid at the same
984 time by using POSIX::setgid() (use only a single numeric argument).
985 Changes to $) require a check to $! to detect any possible errors
986 after an attempted change.
988 (Mnemonic: parentheses are used to I<group> things. The effective gid
989 is the group that's I<right> for you, if you're running setgid.)
991 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
992 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
993 and C<$)> can be swapped only on machines supporting setregid().
998 X<$0> X<$PROGRAM_NAME>
1000 Contains the name of the program being executed.
1002 On some (read: not all) operating systems assigning to C<$0> modifies
1003 the argument area that the C<ps> program sees. On some platforms you
1004 may have to use special C<ps> options or a different C<ps> to see the
1005 changes. Modifying the $0 is more useful as a way of indicating the
1006 current program state than it is for hiding the program you're
1007 running. (Mnemonic: same as B<sh> and B<ksh>.)
1009 Note that there are platform specific limitations on the maximum
1010 length of C<$0>. In the most extreme case it may be limited to the
1011 space occupied by the original C<$0>.
1013 In some platforms there may be arbitrary amount of padding, for
1014 example space characters, after the modified name as shown by C<ps>.
1015 In some platforms this padding may extend all the way to the original
1016 length of the argument area, no matter what you do (this is the case
1017 for example with Linux 2.2).
1019 Note for BSD users: setting C<$0> does not completely remove "perl"
1020 from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
1021 result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
1022 and the " (perl)" suffix are shown depends on your exact BSD variant
1023 and version). This is an operating system feature, Perl cannot help it.
1025 In multithreaded scripts Perl coordinates the threads so that any
1026 thread may modify its copy of the C<$0> and the change becomes visible
1027 to ps(1) (assuming the operating system plays along). Note that
1028 the view of C<$0> the other threads have will not change since they
1029 have their own copies of it.
1031 If the program has been given to perl via the switches C<-e> or C<-E>,
1032 C<$0> will contain the string C<"-e">.
1037 The index of the first element in an array, and of the first character
1038 in a substring. Default is 0, but you could theoretically set it
1039 to 1 to make Perl behave more like B<awk> (or Fortran) when
1040 subscripting and when evaluating the index() and substr() functions.
1041 (Mnemonic: [ begins subscripts.)
1043 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
1044 directive, and cannot influence the behavior of any other file.
1045 (That's why you can only assign compile-time constants to it.)
1046 Its use is highly discouraged.
1048 Note that, unlike other compile-time directives (such as L<strict>),
1049 assignment to C<$[> can be seen from outer lexical scopes in the same file.
1050 However, you can use local() on it to strictly bind its value to a
1056 The version + patchlevel / 1000 of the Perl interpreter. This variable
1057 can be used to determine whether the Perl interpreter executing a
1058 script is in the right range of versions. (Mnemonic: Is this version
1059 of perl in the right bracket?) Example:
1061 warn "No checksumming!\n" if $] < 3.019;
1063 See also the documentation of C<use VERSION> and C<require VERSION>
1064 for a convenient way to fail if the running Perl interpreter is too old.
1066 The floating point representation can sometimes lead to inaccurate
1067 numeric comparisons. See C<$^V> for a more modern representation of
1068 the Perl version that allows accurate string comparisons.
1073 X<$^C> X<$COMPILING>
1075 The current value of the flag associated with the B<-c> switch.
1076 Mainly of use with B<-MO=...> to allow code to alter its behavior
1077 when being compiled, such as for example to AUTOLOAD at compile
1078 time rather than normal, deferred loading. Setting
1079 C<$^C = 1> is similar to calling C<B::minus_c>.
1084 X<$^D> X<$DEBUGGING>
1086 The current value of the debugging flags. (Mnemonic: value of B<-D>
1087 switch.) May be read or set. Like its command-line equivalent, you can use
1088 numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
1090 =item ${^RE_DEBUG_FLAGS}
1092 The current value of the regex debugging flags. Set to 0 for no debug output
1093 even when the re 'debug' module is loaded. See L<re> for details.
1095 =item ${^RE_TRIE_MAXBUF}
1097 Controls how certain regex optimisations are applied and how much memory they
1098 utilize. This value by default is 65536 which corresponds to a 512kB temporary
1099 cache. Set this to a higher value to trade memory for speed when matching
1100 large alternations. Set it to a lower value if you want the optimisations to
1101 be as conservative of memory as possible but still occur, and set it to a
1102 negative value to prevent the optimisation and conserve the most memory.
1103 Under normal situations this variable should be of no interest to you.
1105 =item $SYSTEM_FD_MAX
1108 X<$^F> X<$SYSTEM_FD_MAX>
1110 The maximum system file descriptor, ordinarily 2. System file
1111 descriptors are passed to exec()ed processes, while higher file
1112 descriptors are not. Also, during an open(), system file descriptors are
1113 preserved even if the open() fails. (Ordinary file descriptors are
1114 closed before the open() is attempted.) The close-on-exec
1115 status of a file descriptor will be decided according to the value of
1116 C<$^F> when the corresponding file, pipe, or socket was opened, not the
1121 WARNING: This variable is strictly for internal use only. Its availability,
1122 behavior, and contents are subject to change without notice.
1124 This variable contains compile-time hints for the Perl interpreter. At the
1125 end of compilation of a BLOCK the value of this variable is restored to the
1126 value when the interpreter started to compile the BLOCK.
1128 When perl begins to parse any block construct that provides a lexical scope
1129 (e.g., eval body, required file, subroutine body, loop body, or conditional
1130 block), the existing value of $^H is saved, but its value is left unchanged.
1131 When the compilation of the block is completed, it regains the saved value.
1132 Between the points where its value is saved and restored, code that
1133 executes within BEGIN blocks is free to change the value of $^H.
1135 This behavior provides the semantic of lexical scoping, and is used in,
1136 for instance, the C<use strict> pragma.
1138 The contents should be an integer; different bits of it are used for
1139 different pragmatic flags. Here's an example:
1141 sub add_100 { $^H |= 0x100 }
1148 Consider what happens during execution of the BEGIN block. At this point
1149 the BEGIN block has already been compiled, but the body of foo() is still
1150 being compiled. The new value of $^H will therefore be visible only while
1151 the body of foo() is being compiled.
1153 Substitution of the above BEGIN block with:
1155 BEGIN { require strict; strict->import('vars') }
1157 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1158 version of the same lexical pragma:
1160 BEGIN { require strict; strict->import('vars') if $condition }
1164 The %^H hash provides the same scoping semantic as $^H. This makes it
1165 useful for implementation of lexically scoped pragmas. See L<perlpragma>.
1170 X<$^I> X<$INPLACE_EDIT>
1172 The current value of the inplace-edit extension. Use C<undef> to disable
1173 inplace editing. (Mnemonic: value of B<-i> switch.)
1178 By default, running out of memory is an untrappable, fatal error.
1179 However, if suitably built, Perl can use the contents of C<$^M>
1180 as an emergency memory pool after die()ing. Suppose that your Perl
1181 were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
1184 $^M = 'a' x (1 << 16);
1186 would allocate a 64K buffer for use in an emergency. See the
1187 F<INSTALL> file in the Perl distribution for information on how to
1188 add custom C compilation flags when compiling perl. To discourage casual
1189 use of this advanced feature, there is no L<English|English> long name for
1197 The name of the operating system under which this copy of Perl was
1198 built, as determined during the configuration process. The value
1199 is identical to C<$Config{'osname'}>. See also L<Config> and the
1200 B<-V> command-line switch documented in L<perlrun>.
1202 In Windows platforms, $^O is not very helpful: since it is always
1203 C<MSWin32>, it doesn't tell the difference between
1204 95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1205 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1206 between the variants.
1210 An internal variable used by PerlIO. A string in two parts, separated
1211 by a C<\0> byte, the first part describes the input layers, the second
1212 part describes the output layers.
1219 The internal variable for debugging support. The meanings of the
1220 various bits are subject to change, but currently indicate:
1226 Debug subroutine enter/exit.
1230 Line-by-line debugging. Causes DB::DB() subroutine to be called for each
1231 statement executed. Also causes saving source code lines (like 0x400).
1235 Switch off optimizations.
1239 Preserve more data for future interactive inspections.
1243 Keep info about source lines on which a subroutine is defined.
1247 Start with single-step on.
1251 Use subroutine address instead of name when reporting.
1255 Report C<goto &subroutine> as well.
1259 Provide informative "file" names for evals based on the place they were compiled.
1263 Provide informative names to anonymous subroutines based on the place they
1268 Save source code lines into C<@{"_<$filename"}>.
1272 Some bits may be relevant at compile-time only, some at
1273 run-time only. This is a new mechanism and the details may change.
1274 See also L<perldebguts>.
1276 =item $LAST_REGEXP_CODE_RESULT
1279 X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1281 The result of evaluation of the last successful C<(?{ code })>
1282 regular expression assertion (see L<perlre>). May be written to.
1284 =item $EXCEPTIONS_BEING_CAUGHT
1287 X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1289 Current state of the interpreter.
1292 --------- -------------------
1293 undef Parsing module/eval
1294 true (1) Executing an eval
1297 The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
1304 The time at which the program began running, in seconds since the
1305 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
1306 and B<-C> filetests are based on this value.
1310 Reflects if taint mode is on or off. 1 for on (the program was run with
1311 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
1312 B<-t> or B<-TU>). This variable is read-only.
1316 Reflects certain Unicode settings of Perl. See L<perlrun>
1317 documentation for the C<-C> switch for more information about
1318 the possible values. This variable is set during Perl startup
1319 and is thereafter read-only.
1323 This variable controls the state of the internal UTF-8 offset caching code.
1324 1 for on (the default), 0 for off, -1 to debug the caching code by checking
1325 all its results against linear scans, and panicking on any discrepancy.
1327 =item ${^UTF8LOCALE}
1329 This variable indicates whether an UTF-8 locale was detected by perl at
1330 startup. This information is used by perl when it's in
1331 adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
1332 switch); see L<perlrun> for more info on this.
1337 X<$^V> X<$PERL_VERSION>
1339 The revision, version, and subversion of the Perl interpreter, represented
1340 as a C<version> object.
1342 This variable first appeared in perl 5.6.0; earlier versions of perl will
1343 see an undefined value. Before perl 5.10.0 $^V was represented as a v-string.
1345 $^V can be used to determine whether the Perl interpreter executing a
1346 script is in the right range of versions. (Mnemonic: use ^V for Version
1349 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
1351 To convert C<$^V> into its string representation use sprintf()'s
1352 C<"%vd"> conversion:
1354 printf "version is v%vd\n", $^V; # Perl's version
1356 See the documentation of C<use VERSION> and C<require VERSION>
1357 for a convenient way to fail if the running Perl interpreter is too old.
1359 See also C<$]> for an older representation of the Perl version.
1366 The current value of the warning switch, initially true if B<-w>
1367 was used, false otherwise, but directly modifiable. (Mnemonic:
1368 related to the B<-w> switch.) See also L<warnings>.
1370 =item ${^WARNING_BITS}
1372 The current set of warning checks enabled by the C<use warnings> pragma.
1373 See the documentation of C<warnings> for more details.
1375 =item ${^WIN32_SLOPPY_STAT}
1377 If this variable is set to a true value, then stat() on Windows will
1378 not try to open the file. This means that the link count cannot be
1379 determined and file attributes may be out of date if additional
1380 hardlinks to the file exist. On the other hand, not opening the file
1381 is considerably faster, especially for files on network drives.
1383 This variable could be set in the F<sitecustomize.pl> file to
1384 configure the local Perl installation to use "sloppy" stat() by
1385 default. See L<perlrun> for more information about site
1388 =item $EXECUTABLE_NAME
1391 X<$^X> X<$EXECUTABLE_NAME>
1393 The name used to execute the current copy of Perl, from C's
1394 C<argv[0]> or (where supported) F</proc/self/exe>.
1396 Depending on the host operating system, the value of $^X may be
1397 a relative or absolute pathname of the perl program file, or may
1398 be the string used to invoke perl but not the pathname of the
1399 perl program file. Also, most operating systems permit invoking
1400 programs that are not in the PATH environment variable, so there
1401 is no guarantee that the value of $^X is in PATH. For VMS, the
1402 value may or may not include a version number.
1404 You usually can use the value of $^X to re-invoke an independent
1405 copy of the same perl that is currently running, e.g.,
1407 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1409 But recall that not all operating systems support forking or
1410 capturing of the output of commands, so this complex statement
1411 may not be portable.
1413 It is not safe to use the value of $^X as a path name of a file,
1414 as some operating systems that have a mandatory suffix on
1415 executable files do not require use of the suffix when invoking
1416 a command. To convert the value of $^X to a path name, use the
1417 following statements:
1419 # Build up a set of file names (not command names).
1423 {$this_perl .= $Config{_exe}
1424 unless $this_perl =~ m/$Config{_exe}$/i;}
1426 Because many operating systems permit anyone with read access to
1427 the Perl program file to make a copy of it, patch the copy, and
1428 then execute the copy, the security-conscious Perl programmer
1429 should take care to invoke the installed copy of perl, not the
1430 copy referenced by $^X. The following statements accomplish
1431 this goal, and produce a pathname that can be invoked as a
1432 command or referenced as a file.
1435 $secure_perl_path = $Config{perlpath};
1437 {$secure_perl_path .= $Config{_exe}
1438 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
1443 The special filehandle that iterates over command-line filenames in
1444 C<@ARGV>. Usually written as the null filehandle in the angle operator
1445 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1446 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1447 corresponding to the last file opened by C<< <> >>. In particular,
1448 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1449 may not cause your function to automatically read the contents of all the
1455 contains the name of the current file when reading from <>.
1460 The array @ARGV contains the command-line arguments intended for
1461 the script. C<$#ARGV> is generally the number of arguments minus
1462 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1463 command name itself. See C<$0> for the command name.
1468 The special filehandle that points to the currently open output file
1469 when doing edit-in-place processing with B<-i>. Useful when you have
1470 to do a lot of inserting and don't want to keep modifying $_. See
1471 L<perlrun> for the B<-i> switch.
1476 The array @F contains the fields of each line read in when autosplit
1477 mode is turned on. See L<perlrun> for the B<-a> switch. This array
1478 is package-specific, and must be declared or given a full package name
1479 if not in package main when running under C<strict 'vars'>.
1484 The array @INC contains the list of places that the C<do EXPR>,
1485 C<require>, or C<use> constructs look for their library files. It
1486 initially consists of the arguments to any B<-I> command-line
1487 switches, followed by the default Perl library, probably
1488 F</usr/local/lib/perl>, followed by ".", to represent the current
1489 directory. ("." will not be appended if taint checks are enabled, either by
1490 C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
1491 the C<use lib> pragma to get the machine-dependent library properly
1494 use lib '/mypath/libdir/';
1497 You can also insert hooks into the file inclusion system by putting Perl
1498 code directly into @INC. Those hooks may be subroutine references, array
1499 references or blessed objects. See L<perlfunc/require> for details.
1506 Within a subroutine the array @_ contains the parameters passed to that
1507 subroutine. See L<perlsub>.
1512 The hash %INC contains entries for each filename included via the
1513 C<do>, C<require>, or C<use> operators. The key is the filename
1514 you specified (with module names converted to pathnames), and the
1515 value is the location of the file found. The C<require>
1516 operator uses this hash to determine whether a particular file has
1517 already been included.
1519 If the file was loaded via a hook (e.g. a subroutine reference, see
1520 L<perlfunc/require> for a description of these hooks), this hook is
1521 by default inserted into %INC in place of a filename. Note, however,
1522 that the hook may have set the %INC entry by itself to provide some more
1530 The hash %ENV contains your current environment. Setting a
1531 value in C<ENV> changes the environment for any child processes
1532 you subsequently fork() off.
1539 The hash C<%SIG> contains signal handlers for signals. For example:
1541 sub handler { # 1st argument is signal name
1543 print "Caught a SIG$sig--shutting down\n";
1548 $SIG{'INT'} = \&handler;
1549 $SIG{'QUIT'} = \&handler;
1551 $SIG{'INT'} = 'DEFAULT'; # restore default action
1552 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1554 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1555 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1558 Here are some other examples:
1560 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1561 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1562 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1563 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1565 Be sure not to use a bareword as the name of a signal handler,
1566 lest you inadvertently call it.
1568 If your system has the sigaction() function then signal handlers are
1569 installed using it. This means you get reliable signal handling.
1571 The default delivery policy of signals changed in Perl 5.8.0 from
1572 immediate (also known as "unsafe") to deferred, also known as
1573 "safe signals". See L<perlipc> for more information.
1575 Certain internal hooks can be also set using the %SIG hash. The
1576 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1577 about to be printed. The warning message is passed as the first
1578 argument. The presence of a C<__WARN__> hook causes the ordinary printing
1579 of warnings to C<STDERR> to be suppressed. You can use this to save warnings
1580 in a variable, or turn warnings into fatal errors, like this:
1582 local $SIG{__WARN__} = sub { die $_[0] };
1585 As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
1586 disable warnings using the empty subroutine:
1588 local $SIG{__WARN__} = sub {};
1590 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1591 is about to be thrown. The error message is passed as the first
1592 argument. When a C<__DIE__> hook routine returns, the exception
1593 processing continues as it would have in the absence of the hook,
1594 unless the hook routine itself exits via a C<goto>, a loop exit, or a C<die()>.
1595 The C<__DIE__> handler is explicitly disabled during the call, so that you
1596 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1598 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1599 even inside an eval(). Do not use this to rewrite a pending exception
1600 in C<$@>, or as a bizarre substitute for overriding C<CORE::GLOBAL::die()>.
1601 This strange action at a distance may be fixed in a future release
1602 so that C<$SIG{__DIE__}> is only called if your program is about
1603 to exit, as was the original intent. Any other use is deprecated.
1605 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1606 they may be called to report (probable) errors found by the parser.
1607 In such a case the parser may be in inconsistent state, so any
1608 attempt to evaluate Perl code from such a handler will probably
1609 result in a segfault. This means that warnings or errors that
1610 result from parsing Perl should be used with extreme caution, like
1613 require Carp if defined $^S;
1614 Carp::confess("Something wrong") if defined &Carp::confess;
1615 die "Something wrong, but could not load Carp to give backtrace...
1616 To see backtrace try starting Perl with -MCarp switch";
1618 Here the first line will load Carp I<unless> it is the parser who
1619 called the handler. The second line will print backtrace and die if
1620 Carp was available. The third line will be executed only if Carp was
1623 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1624 L<warnings> for additional information.
1628 =head2 Error Indicators
1629 X<error> X<exception>
1631 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1632 about different types of error conditions that may appear during
1633 execution of a Perl program. The variables are shown ordered by
1634 the "distance" between the subsystem which reported the error and
1635 the Perl process. They correspond to errors detected by the Perl
1636 interpreter, C library, operating system, or an external program,
1639 To illustrate the differences between these variables, consider the
1640 following Perl expression, which uses a single-quoted string:
1643 open my $pipe, "/cdrom/install |" or die $!;
1645 close $pipe or die "bad pipe: $?, $!";
1648 After execution of this statement all 4 variables may have been set.
1650 C<$@> is set if the string to be C<eval>-ed did not compile (this
1651 may happen if C<open> or C<close> were imported with bad prototypes),
1652 or if Perl code executed during evaluation die()d . In these cases
1653 the value of $@ is the compile error, or the argument to C<die>
1654 (which will interpolate C<$!> and C<$?>). (See also L<Fatal>,
1657 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1658 and C<close> are translated to calls in the C run-time library and
1659 thence to the operating system kernel. C<$!> is set to the C library's
1660 C<errno> if one of these calls fails.
1662 Under a few operating systems, C<$^E> may contain a more verbose
1663 error indicator, such as in this case, "CDROM tray not closed."
1664 Systems that do not support extended error messages leave C<$^E>
1667 Finally, C<$?> may be set to non-0 value if the external program
1668 F</cdrom/install> fails. The upper eight bits reflect specific
1669 error conditions encountered by the program (the program's exit()
1670 value). The lower eight bits reflect mode of failure, like signal
1671 death and core dump information See wait(2) for details. In
1672 contrast to C<$!> and C<$^E>, which are set only if error condition
1673 is detected, the variable C<$?> is set on each C<wait> or pipe
1674 C<close>, overwriting the old value. This is more like C<$@>, which
1675 on every eval() is always set on failure and cleared on success.
1677 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1680 =head2 Technical Note on the Syntax of Variable Names
1682 Variable names in Perl can have several formats. Usually, they
1683 must begin with a letter or underscore, in which case they can be
1684 arbitrarily long (up to an internal limit of 251 characters) and
1685 may contain letters, digits, underscores, or the special sequence
1686 C<::> or C<'>. In this case, the part before the last C<::> or
1687 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1689 Perl variable names may also be a sequence of digits or a single
1690 punctuation or control character. These names are all reserved for
1691 special uses by Perl; for example, the all-digits names are used
1692 to hold data captured by backreferences after a regular expression
1693 match. Perl has a special syntax for the single-control-character
1694 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1695 character. For example, the notation C<$^W> (dollar-sign caret
1696 C<W>) is the scalar variable whose name is the single character
1697 control-C<W>. This is better than typing a literal control-C<W>
1700 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1701 strings that begin with control characters (or better yet, a caret).
1702 These variables must be written in the form C<${^Foo}>; the braces
1703 are not optional. C<${^Foo}> denotes the scalar variable whose
1704 name is a control-C<F> followed by two C<o>'s. These variables are
1705 reserved for future special uses by Perl, except for the ones that
1706 begin with C<^_> (control-underscore or caret-underscore). No
1707 control-character name that begins with C<^_> will acquire a special
1708 meaning in any future version of Perl; such names may therefore be
1709 used safely in programs. C<$^_> itself, however, I<is> reserved.
1711 Perl identifiers that begin with digits, control characters, or
1712 punctuation characters are exempt from the effects of the C<package>
1713 declaration and are always forced to be in package C<main>; they are
1714 also exempt from C<strict 'vars'> errors. A few other names are also
1715 exempt in these ways:
1723 In particular, the new special C<${^_XYZ}> variables are always taken
1724 to be in package C<main>, regardless of any C<package> declarations
1729 Due to an unfortunate accident of Perl's implementation, C<use
1730 English> imposes a considerable performance penalty on all regular
1731 expression matches in a program, regardless of whether they occur
1732 in the scope of C<use English>. For that reason, saying C<use
1733 English> in libraries is strongly discouraged. See the
1734 Devel::SawAmpersand module documentation from CPAN
1735 ( http://www.cpan.org/modules/by-module/Devel/ )
1736 for more information. Writing C<use English '-no_match_vars';>
1737 avoids the performance penalty.
1739 Having to even think about the C<$^S> variable in your exception
1740 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1741 invites grievous and difficult to track down errors. Avoid it
1742 and use an C<END{}> or CORE::GLOBAL::die override instead.