3 perlvar - Perl predefined variables
7 =head2 Predefined Names
9 The following names have special meaning to Perl. Most
10 punctuation names have reasonable mnemonics, or analogs in the
11 shells. Nevertheless, if you wish to use long variable names,
16 at the top of your program. This will alias all the short names to the
17 long names in the current package. Some even have medium names,
18 generally borrowed from B<awk>.
20 If you don't mind the performance hit, variables that depend on the
21 currently selected filehandle may instead be set by calling an
22 appropriate object method on the IO::Handle object. (Summary lines
23 below for this contain the word HANDLE.) First you must say
27 after which you may use either
35 Each method returns the old value of the IO::Handle attribute.
36 The methods each take an optional EXPR, which if supplied specifies the
37 new value for the IO::Handle attribute in question. If not supplied,
38 most methods do nothing to the current value--except for
39 autoflush(), which will assume a 1 for you, just to be different.
40 Because loading in the IO::Handle class is an expensive operation, you should
41 learn how to use the regular built-in variables.
43 A few of these variables are considered "read-only". This means that if
44 you try to assign to this variable, either directly or indirectly through
45 a reference, you'll raise a run-time exception.
47 The following list is ordered by scalar variables first, then the
48 arrays, then the hashes.
56 The default input and pattern-searching space. The following pairs are
59 while (<>) {...} # equivalent only in while!
60 while (defined($_ = <>)) {...}
71 Here are the places where Perl will assume $_ even if you
78 Various unary functions, including functions like ord() and int(), as well
79 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
84 Various list functions like print() and unlink().
88 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
89 without an C<=~> operator.
93 The default iterator variable in a C<foreach> loop if no other
98 The implicit iterator variable in the grep() and map() functions.
102 The default place to put an input record when a C<< <FH> >>
103 operation's result is tested by itself as the sole criterion of a C<while>
104 test. Outside a C<while> test, this will not happen.
108 (Mnemonic: underline is understood in certain operations.)
116 Contains the subpattern from the corresponding set of capturing
117 parentheses from the last pattern match, not counting patterns
118 matched in nested blocks that have been exited already. (Mnemonic:
119 like \digits.) These variables are all read-only and dynamically
120 scoped to the current BLOCK.
126 The string matched by the last successful pattern match (not counting
127 any matches hidden within a BLOCK or eval() enclosed by the current
128 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
129 and dynamically scoped to the current BLOCK.
131 The use of this variable anywhere in a program imposes a considerable
132 performance penalty on all regular expression matches. See L<BUGS>.
138 The string preceding whatever was matched by the last successful
139 pattern match (not counting any matches hidden within a BLOCK or eval
140 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
141 string.) This variable is read-only.
143 The use of this variable anywhere in a program imposes a considerable
144 performance penalty on all regular expression matches. See L<BUGS>.
150 The string following whatever was matched by the last successful
151 pattern match (not counting any matches hidden within a BLOCK or eval()
152 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
157 print "$`:$&:$'\n"; # prints abc:def:ghi
159 This variable is read-only and dynamically scoped to the current BLOCK.
161 The use of this variable anywhere in a program imposes a considerable
162 performance penalty on all regular expression matches. See L<BUGS>.
164 =item $LAST_PAREN_MATCH
168 The last bracket matched by the last search pattern. This is useful if
169 you don't know which one of a set of alternative patterns matched. For
172 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
174 (Mnemonic: be positive and forward looking.)
175 This variable is read-only and dynamically scoped to the current BLOCK.
177 =item @LAST_MATCH_END
181 This array holds the offsets of the ends of the last successful
182 submatches in the currently active dynamic scope. C<$+[0]> is
183 the offset into the string of the end of the entire match. This
184 is the same value as what the C<pos> function returns when called
185 on the variable that was matched against. The I<n>th element
186 of this array holds the offset of the I<n>th submatch, so
187 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
188 past where $2 ends, and so on. You can use C<$#+> to determine
189 how many subgroups were in the last successful match. See the
190 examples given for the C<@-> variable.
192 =item $MULTILINE_MATCHING
196 Set to 1 to do multi-line matching within a string, 0 to tell Perl
197 that it can assume that strings contain a single line, for the purpose
198 of optimizing pattern matches. Pattern matches on strings containing
199 multiple newlines can produce confusing results when C<$*> is 0. Default
200 is 0. (Mnemonic: * matches multiple things.) This variable
201 influences the interpretation of only C<^> and C<$>. A literal newline can
202 be searched for even when C<$* == 0>.
204 Use of C<$*> is deprecated in modern Perl, supplanted by
205 the C</s> and C</m> modifiers on pattern matching.
207 =item input_line_number HANDLE EXPR
209 =item $INPUT_LINE_NUMBER
215 The current input record number for the last file handle from which
216 you just read() (or called a C<seek> or C<tell> on). The value
217 may be different from the actual physical line number in the file,
218 depending on what notion of "line" is in effect--see C<$/> on how
219 to change that. An explicit close on a filehandle resets the line
220 number. Because C<< <> >> never does an explicit close, line
221 numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
222 Consider this variable read-only: setting it does not reposition
223 the seek pointer; you'll have to do that on your own. Localizing C<$.>
224 has the effect of also localizing Perl's notion of "the last read
225 filehandle". (Mnemonic: many programs use "." to mean the current line
228 =item input_record_separator HANDLE EXPR
230 =item $INPUT_RECORD_SEPARATOR
236 The input record separator, newline by default. This
237 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
238 variable, including treating empty lines as a terminator if set to
239 the null string. (An empty line cannot contain any spaces
240 or tabs.) You may set it to a multi-character string to match a
241 multi-character terminator, or to C<undef> to read through the end
242 of file. Setting it to C<"\n\n"> means something slightly
243 different than setting to C<"">, if the file contains consecutive
244 empty lines. Setting to C<""> will treat two or more consecutive
245 empty lines as a single empty line. Setting to C<"\n\n"> will
246 blindly assume that the next input character belongs to the next
247 paragraph, even if it's a newline. (Mnemonic: / delimits
248 line boundaries when quoting poetry.)
250 undef $/; # enable "slurp" mode
251 $_ = <FH>; # whole file now here
254 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
255 better for something. :-)
257 Setting C<$/> to a reference to an integer, scalar containing an integer, or
258 scalar that's convertible to an integer will attempt to read records
259 instead of lines, with the maximum record size being the referenced
262 $/ = \32768; # or \"32768", or \$var_containing_32768
266 will read a record of no more than 32768 bytes from FILE. If you're
267 not reading from a record-oriented file (or your OS doesn't have
268 record-oriented files), then you'll likely get a full chunk of data
269 with every read. If a record is larger than the record size you've
270 set, you'll get the record back in pieces.
272 On VMS, record reads are done with the equivalent of C<sysread>,
273 so it's best not to mix record and non-record reads on the same
274 file. (This is unlikely to be a problem, because any file you'd
275 want to read in record mode is probably unusable in line mode.)
276 Non-VMS systems do normal I/O, so it's safe to mix record and
277 non-record reads of a file.
279 See also L<perlport/"Newlines">. Also see C<$.>.
281 =item autoflush HANDLE EXPR
283 =item $OUTPUT_AUTOFLUSH
287 If set to nonzero, forces a flush right away and after every write
288 or print on the currently selected output channel. Default is 0
289 (regardless of whether the channel is really buffered by the
290 system or not; C<$|> tells you only whether you've asked Perl
291 explicitly to flush after each write). STDOUT will
292 typically be line buffered if output is to the terminal and block
293 buffered otherwise. Setting this variable is useful primarily when
294 you are outputting to a pipe or socket, such as when you are running
295 a Perl program under B<rsh> and want to see the output as it's
296 happening. This has no effect on input buffering. See L<perlfunc/getc>
297 for that. (Mnemonic: when you want your pipes to be piping hot.)
299 =item output_field_separator HANDLE EXPR
301 =item $OUTPUT_FIELD_SEPARATOR
307 The output field separator for the print operator. Ordinarily the
308 print operator simply prints out its arguments without further
309 adornment. To get behavior more like B<awk>, set this variable as
310 you would set B<awk>'s OFS variable to specify what is printed
311 between fields. (Mnemonic: what is printed when there is a "," in
312 your print statement.)
314 =item output_record_separator HANDLE EXPR
316 =item $OUTPUT_RECORD_SEPARATOR
322 The output record separator for the print operator. Ordinarily the
323 print operator simply prints out its arguments as is, with no
324 trailing newline or other end-of-record string added. To get
325 behavior more like B<awk>, set this variable as you would set
326 B<awk>'s ORS variable to specify what is printed at the end of the
327 print. (Mnemonic: you set C<$\> instead of adding "\n" at the
328 end of the print. Also, it's just like C<$/>, but it's what you
329 get "back" from Perl.)
331 =item $LIST_SEPARATOR
335 This is like C<$,> except that it applies to array and slice values
336 interpolated into a double-quoted string (or similar interpreted
337 string). Default is a space. (Mnemonic: obvious, I think.)
339 =item $SUBSCRIPT_SEPARATOR
345 The subscript separator for multidimensional array emulation. If you
346 refer to a hash element as
352 $foo{join($;, $a, $b, $c)}
356 @foo{$a,$b,$c} # a slice--note the @
360 ($foo{$a},$foo{$b},$foo{$c})
362 Default is "\034", the same as SUBSEP in B<awk>. If your
363 keys contain binary data there might not be any safe value for C<$;>.
364 (Mnemonic: comma (the syntactic subscript separator) is a
365 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
366 taken for something more important.)
368 Consider using "real" multidimensional arrays as described
375 The output format for printed numbers. This variable is a half-hearted
376 attempt to emulate B<awk>'s OFMT variable. There are times, however,
377 when B<awk> and Perl have differing notions of what counts as
378 numeric. The initial value is "%.I<n>g", where I<n> is the value
379 of the macro DBL_DIG from your system's F<float.h>. This is different from
380 B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
381 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
383 Use of C<$#> is deprecated.
385 =item format_page_number HANDLE EXPR
387 =item $FORMAT_PAGE_NUMBER
391 The current page number of the currently selected output channel.
393 (Mnemonic: % is page number in B<nroff>.)
395 =item format_lines_per_page HANDLE EXPR
397 =item $FORMAT_LINES_PER_PAGE
401 The current page length (printable lines) of the currently selected
402 output channel. Default is 60.
404 (Mnemonic: = has horizontal lines.)
406 =item format_lines_left HANDLE EXPR
408 =item $FORMAT_LINES_LEFT
412 The number of lines left on the page of the currently selected output
415 (Mnemonic: lines_on_page - lines_printed.)
417 =item @LAST_MATCH_START
421 $-[0] is the offset of the start of the last successful match.
422 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
423 I<n>-th subpattern, or undef if the subpattern did not match.
425 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
426 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
427 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
428 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
429 matched subgroup in the last successful match. Contrast with
430 C<$#+>, the number of subgroups in the regular expression. Compare
433 This array holds the offsets of the beginnings of the last
434 successful submatches in the currently active dynamic scope.
435 C<$-[0]> is the offset into the string of the beginning of the
436 entire match. The I<n>th element of this array holds the offset
437 of the I<n>th submatch, so C<$+[1]> is the offset where $1
438 begins, C<$+[2]> the offset where $2 begins, and so on.
439 You can use C<$#-> to determine how many subgroups were in the
440 last successful match. Compare with the C<@+> variable.
442 After a match against some variable $var:
446 =item C<$`> is the same as C<substr($var, 0, $-[0])>
448 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
450 =item C<$'> is the same as C<substr($var, $+[0])>
452 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
454 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
456 =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
460 =item format_name HANDLE EXPR
466 The name of the current report format for the currently selected output
467 channel. Default is the name of the filehandle. (Mnemonic: brother to
470 =item format_top_name HANDLE EXPR
472 =item $FORMAT_TOP_NAME
476 The name of the current top-of-page format for the currently selected
477 output channel. Default is the name of the filehandle with _TOP
478 appended. (Mnemonic: points to top of page.)
480 =item format_line_break_characters HANDLE EXPR
482 =item $FORMAT_LINE_BREAK_CHARACTERS
486 The current set of characters after which a string may be broken to
487 fill continuation fields (starting with ^) in a format. Default is
488 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
489 poetry is a part of a line.)
491 =item format_formfeed HANDLE EXPR
493 =item $FORMAT_FORMFEED
497 What formats output as a form feed. Default is \f.
503 The current value of the write() accumulator for format() lines. A format
504 contains formline() calls that put their result into C<$^A>. After
505 calling its format, write() prints out the contents of C<$^A> and empties.
506 So you never really see the contents of C<$^A> unless you call
507 formline() yourself and then look at it. See L<perlform> and
508 L<perlfunc/formline()>.
514 The status returned by the last pipe close, backtick (C<``>) command,
515 successful call to wait() or waitpid(), or from the system()
516 operator. This is just the 16-bit status word returned by the
517 wait() system call (or else is made up to look like it). Thus, the
518 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
519 C<$? & 127> gives which signal, if any, the process died from, and
520 C<$? & 128> reports whether there was a core dump. (Mnemonic:
521 similar to B<sh> and B<ksh>.)
523 Additionally, if the C<h_errno> variable is supported in C, its value
524 is returned via $? if any C<gethost*()> function fails.
526 If you have installed a signal handler for C<SIGCHLD>, the
527 value of C<$?> will usually be wrong outside that handler.
529 Inside an C<END> subroutine C<$?> contains the value that is going to be
530 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
531 change the exit status of your program. For example:
534 $? = 1 if $? == 255; # die would make it 255
537 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
538 actual VMS exit status, instead of the default emulation of POSIX
541 Also see L<Error Indicators>.
549 If used numerically, yields the current value of the C C<errno>
550 variable, with all the usual caveats. (This means that you shouldn't
551 depend on the value of C<$!> to be anything in particular unless
552 you've gotten a specific error return indicating a system error.)
553 If used an a string, yields the corresponding system error string.
554 You can assign a number to C<$!> to set I<errno> if, for instance,
555 you want C<"$!"> to return the string for error I<n>, or you want
556 to set the exit value for the die() operator. (Mnemonic: What just
559 Also see L<Error Indicators>.
561 =item $EXTENDED_OS_ERROR
565 Error information specific to the current operating system. At
566 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
567 (and for MacPerl). On all other platforms, C<$^E> is always just
570 Under VMS, C<$^E> provides the VMS status value from the last
571 system error. This is more specific information about the last
572 system error than that provided by C<$!>. This is particularly
573 important when C<$!> is set to B<EVMSERR>.
575 Under OS/2, C<$^E> is set to the error code of the last call to
576 OS/2 API either via CRT, or directly from perl.
578 Under Win32, C<$^E> always returns the last error information
579 reported by the Win32 call C<GetLastError()> which describes
580 the last error from within the Win32 API. Most Win32-specific
581 code will report errors via C<$^E>. ANSI C and Unix-like calls
582 set C<errno> and so most portable Perl code will report errors
585 Caveats mentioned in the description of C<$!> generally apply to
586 C<$^E>, also. (Mnemonic: Extra error explanation.)
588 Also see L<Error Indicators>.
594 The Perl syntax error message from the last eval() operator. If null, the
595 last eval() parsed and executed correctly (although the operations you
596 invoked may have failed in the normal fashion). (Mnemonic: Where was
597 the syntax error "at"?)
599 Warning messages are not collected in this variable. You can,
600 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
603 Also see L<Error Indicators>.
611 The process number of the Perl running this script. You should
612 consider this variable read-only, although it will be altered
613 across fork() calls. (Mnemonic: same as shells.)
621 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
622 if you're running setuid.)
624 =item $EFFECTIVE_USER_ID
630 The effective uid of this process. Example:
632 $< = $>; # set real to effective uid
633 ($<,$>) = ($>,$<); # swap real and effective uid
635 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
636 C<< $< >> and C<< $> >> can be swapped only on machines
637 supporting setreuid().
645 The real gid of this process. If you are on a machine that supports
646 membership in multiple groups simultaneously, gives a space separated
647 list of groups you are in. The first number is the one returned by
648 getgid(), and the subsequent ones by getgroups(), one of which may be
649 the same as the first number.
651 However, a value assigned to C<$(> must be a single number used to
652 set the real gid. So the value given by C<$(> should I<not> be assigned
653 back to C<$(> without being forced numeric, such as by adding zero.
655 (Mnemonic: parentheses are used to I<group> things. The real gid is the
656 group you I<left>, if you're running setgid.)
658 =item $EFFECTIVE_GROUP_ID
664 The effective gid of this process. If you are on a machine that
665 supports membership in multiple groups simultaneously, gives a space
666 separated list of groups you are in. The first number is the one
667 returned by getegid(), and the subsequent ones by getgroups(), one of
668 which may be the same as the first number.
670 Similarly, a value assigned to C<$)> must also be a space-separated
671 list of numbers. The first number sets the effective gid, and
672 the rest (if any) are passed to setgroups(). To get the effect of an
673 empty list for setgroups(), just repeat the new effective gid; that is,
674 to force an effective gid of 5 and an effectively empty setgroups()
675 list, say C< $) = "5 5" >.
677 (Mnemonic: parentheses are used to I<group> things. The effective gid
678 is the group that's I<right> for you, if you're running setgid.)
680 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
681 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
682 and C<$)> can be swapped only on machines supporting setregid().
688 Contains the name of the program being executed. On some operating
689 systems assigning to C<$0> modifies the argument area that the B<ps>
690 program sees. This is more useful as a way of indicating the current
691 program state than it is for hiding the program you're running.
692 (Mnemonic: same as B<sh> and B<ksh>.)
694 Note for BSD users: setting C<$0> does not completely remove "perl"
695 from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
696 result in C<"perl: foobar (perl)">. This is an operating system
701 The index of the first element in an array, and of the first character
702 in a substring. Default is 0, but you could theoretically set it
703 to 1 to make Perl behave more like B<awk> (or Fortran) when
704 subscripting and when evaluating the index() and substr() functions.
705 (Mnemonic: [ begins subscripts.)
707 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
708 directive, and cannot influence the behavior of any other file.
709 Its use is highly discouraged.
713 The version + patchlevel / 1000 of the Perl interpreter. This variable
714 can be used to determine whether the Perl interpreter executing a
715 script is in the right range of versions. (Mnemonic: Is this version
716 of perl in the right bracket?) Example:
718 warn "No checksumming!\n" if $] < 3.019;
720 See also the documentation of C<use VERSION> and C<require VERSION>
721 for a convenient way to fail if the running Perl interpreter is too old.
723 The use of this variable is deprecated. The floating point representation
724 can sometimes lead to inaccurate numeric comparisons. See C<$^V> for a
725 more modern representation of the Perl version that allows accurate string
732 The current value of the flag associated with the B<-c> switch.
733 Mainly of use with B<-MO=...> to allow code to alter its behavior
734 when being compiled, such as for example to AUTOLOAD at compile
735 time rather than normal, deferred loading. See L<perlcc>. Setting
736 C<$^C = 1> is similar to calling C<B::minus_c>.
742 The current value of the debugging flags. (Mnemonic: value of B<-D>
749 The maximum system file descriptor, ordinarily 2. System file
750 descriptors are passed to exec()ed processes, while higher file
751 descriptors are not. Also, during an open(), system file descriptors are
752 preserved even if the open() fails. (Ordinary file descriptors are
753 closed before the open() is attempted.) The close-on-exec
754 status of a file descriptor will be decided according to the value of
755 C<$^F> when the corresponding file, pipe, or socket was opened, not the
760 WARNING: This variable is strictly for internal use only. Its availability,
761 behavior, and contents are subject to change without notice.
763 This variable contains compile-time hints for the Perl interpreter. At the
764 end of compilation of a BLOCK the value of this variable is restored to the
765 value when the interpreter started to compile the BLOCK.
767 When perl begins to parse any block construct that provides a lexical scope
768 (e.g., eval body, required file, subroutine body, loop body, or conditional
769 block), the existing value of $^H is saved, but its value is left unchanged.
770 When the compilation of the block is completed, it regains the saved value.
771 Between the points where its value is saved and restored, code that
772 executes within BEGIN blocks is free to change the value of $^H.
774 This behavior provides the semantic of lexical scoping, and is used in,
775 for instance, the C<use strict> pragma.
777 The contents should be an integer; different bits of it are used for
778 different pragmatic flags. Here's an example:
780 sub add_100 { $^H |= 0x100 }
787 Consider what happens during execution of the BEGIN block. At this point
788 the BEGIN block has already been compiled, but the body of foo() is still
789 being compiled. The new value of $^H will therefore be visible only while
790 the body of foo() is being compiled.
792 Substitution of the above BEGIN block with:
794 BEGIN { require strict; strict->import('vars') }
796 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
797 version of the same lexical pragma:
799 BEGIN { require strict; strict->import('vars') if $condition }
803 WARNING: This variable is strictly for internal use only. Its availability,
804 behavior, and contents are subject to change without notice.
806 The %^H hash provides the same scoping semantic as $^H. This makes it
807 useful for implementation of lexically scoped pragmas.
813 The current value of the inplace-edit extension. Use C<undef> to disable
814 inplace editing. (Mnemonic: value of B<-i> switch.)
818 By default, running out of memory is an untrappable, fatal error.
819 However, if suitably built, Perl can use the contents of C<$^M>
820 as an emergency memory pool after die()ing. Suppose that your Perl
821 were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
824 $^M = 'a' x (1 << 16);
826 would allocate a 64K buffer for use in an emergency. See the
827 F<INSTALL> file in the Perl distribution for information on how to
828 enable this option. To discourage casual use of this advanced
829 feature, there is no L<English|English> long name for this variable.
835 The name of the operating system under which this copy of Perl was
836 built, as determined during the configuration process. The value
837 is identical to C<$Config{'osname'}>. See also L<Config> and the
838 B<-V> command-line switch documented in L<perlrun>.
844 The internal variable for debugging support. The meanings of the
845 various bits are subject to change, but currently indicate:
851 Debug subroutine enter/exit.
855 Line-by-line debugging.
859 Switch off optimizations.
863 Preserve more data for future interactive inspections.
867 Keep info about source lines on which a subroutine is defined.
871 Start with single-step on.
875 Use subroutine address instead of name when reporting.
879 Report C<goto &subroutine> as well.
883 Provide informative "file" names for evals based on the place they were compiled.
887 Provide informative names to anonymous subroutines based on the place they
892 Some bits may be relevant at compile-time only, some at
893 run-time only. This is a new mechanism and the details may change.
895 =item $LAST_REGEXP_CODE_RESULT
899 The result of evaluation of the last successful C<(?{ code })>
900 regular expression assertion (see L<perlre>). May be written to.
902 =item $EXCEPTIONS_BEING_CAUGHT
906 Current state of the interpreter. Undefined if parsing of the current
907 module/eval is not finished (may happen in $SIG{__DIE__} and
908 $SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
914 The time at which the program began running, in seconds since the
915 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
916 and B<-C> filetests are based on this value.
922 The revision, version, and subversion of the Perl interpreter, represented
923 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
924 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
925 C<$^V eq v5.6.0>. Note that the characters in this string value can
926 potentially be in Unicode range.
928 This can be used to determine whether the Perl interpreter executing a
929 script is in the right range of versions. (Mnemonic: use ^V for Version
932 warn "No "our" declarations!\n" if $^V and $^V lt v5.6.0;
934 See the documentation of C<use VERSION> and C<require VERSION>
935 for a convenient way to fail if the running Perl interpreter is too old.
937 See also C<$]> for an older representation of the Perl version.
943 The current value of the warning switch, initially true if B<-w>
944 was used, false otherwise, but directly modifiable. (Mnemonic:
945 related to the B<-w> switch.) See also L<warnings>.
947 =item ${^WARNING_BITS}
949 The current set of warning checks enabled by the C<use warnings> pragma.
950 See the documentation of C<warnings> for more details.
952 =item ${^WIDE_SYSTEM_CALLS}
954 Global flag that enables system calls made by Perl to use wide character
955 APIs native to the system, if available. This is currently only implemented
956 on the Windows platform.
958 This can also be enabled from the command line using the C<-C> switch.
960 The initial value is typically C<0> for compatibility with Perl versions
961 earlier than 5.6, but may be automatically set to C<1> by Perl if the system
962 provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
964 The C<bytes> pragma always overrides the effect of this flag in the current
965 lexical scope. See L<bytes>.
967 =item $EXECUTABLE_NAME
971 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
972 This may not be a full pathname, nor even necessarily in your path.
976 contains the name of the current file when reading from <>.
980 The array @ARGV contains the command-line arguments intended for
981 the script. C<$#ARGV> is generally the number of arguments minus
982 one, because C<$ARGV[0]> is the first argument, I<not> the program's
983 command name itself. See C<$0> for the command name.
987 The array @INC contains the list of places that the C<do EXPR>,
988 C<require>, or C<use> constructs look for their library files. It
989 initially consists of the arguments to any B<-I> command-line
990 switches, followed by the default Perl library, probably
991 F</usr/local/lib/perl>, followed by ".", to represent the current
992 directory. If you need to modify this at runtime, you should use
993 the C<use lib> pragma to get the machine-dependent library properly
996 use lib '/mypath/libdir/';
1001 Within a subroutine the array @_ contains the parameters passed to that
1002 subroutine. See L<perlsub>.
1006 The hash %INC contains entries for each filename included via the
1007 C<do>, C<require>, or C<use> operators. The key is the filename
1008 you specified (with module names converted to pathnames), and the
1009 value is the location of the file found. The C<require>
1010 operator uses this hash to determine whether a particular file has
1011 already been included.
1017 The hash %ENV contains your current environment. Setting a
1018 value in C<ENV> changes the environment for any child processes
1019 you subsequently fork() off.
1025 The hash %SIG contains signal handlers for signals. For example:
1027 sub handler { # 1st argument is signal name
1029 print "Caught a SIG$sig--shutting down\n";
1034 $SIG{'INT'} = \&handler;
1035 $SIG{'QUIT'} = \&handler;
1037 $SIG{'INT'} = 'DEFAULT'; # restore default action
1038 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1040 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1041 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1044 Here are some other examples:
1046 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1047 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1048 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1049 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1051 Be sure not to use a bareword as the name of a signal handler,
1052 lest you inadvertently call it.
1054 If your system has the sigaction() function then signal handlers are
1055 installed using it. This means you get reliable signal handling. If
1056 your system has the SA_RESTART flag it is used when signals handlers are
1057 installed. This means that system calls for which restarting is supported
1058 continue rather than returning when a signal arrives. If you want your
1059 system calls to be interrupted by signal delivery then do something like
1062 use POSIX ':signal_h';
1065 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1066 or die "Error setting SIGALRM handler: $!\n";
1070 Certain internal hooks can be also set using the %SIG hash. The
1071 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1072 about to be printed. The warning message is passed as the first
1073 argument. The presence of a __WARN__ hook causes the ordinary printing
1074 of warnings to STDERR to be suppressed. You can use this to save warnings
1075 in a variable, or turn warnings into fatal errors, like this:
1077 local $SIG{__WARN__} = sub { die $_[0] };
1080 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1081 is about to be thrown. The error message is passed as the first
1082 argument. When a __DIE__ hook routine returns, the exception
1083 processing continues as it would have in the absence of the hook,
1084 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
1085 The C<__DIE__> handler is explicitly disabled during the call, so that you
1086 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1088 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1089 even inside an eval(). Do not use this to rewrite a pending exception
1090 in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1091 This strange action at a distance may be fixed in a future release
1092 so that C<$SIG{__DIE__}> is only called if your program is about
1093 to exit, as was the original intent. Any other use is deprecated.
1095 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1096 they may be called to report (probable) errors found by the parser.
1097 In such a case the parser may be in inconsistent state, so any
1098 attempt to evaluate Perl code from such a handler will probably
1099 result in a segfault. This means that warnings or errors that
1100 result from parsing Perl should be used with extreme caution, like
1103 require Carp if defined $^S;
1104 Carp::confess("Something wrong") if defined &Carp::confess;
1105 die "Something wrong, but could not load Carp to give backtrace...
1106 To see backtrace try starting Perl with -MCarp switch";
1108 Here the first line will load Carp I<unless> it is the parser who
1109 called the handler. The second line will print backtrace and die if
1110 Carp was available. The third line will be executed only if Carp was
1113 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1114 L<warnings> for additional information.
1118 =head2 Error Indicators
1120 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1121 about different types of error conditions that may appear during
1122 execution of a Perl program. The variables are shown ordered by
1123 the "distance" between the subsystem which reported the error and
1124 the Perl process. They correspond to errors detected by the Perl
1125 interpreter, C library, operating system, or an external program,
1128 To illustrate the differences between these variables, consider the
1129 following Perl expression, which uses a single-quoted string:
1132 open PIPE, "/cdrom/install |";
1134 close PIPE or die "bad pipe: $?, $!";
1137 After execution of this statement all 4 variables may have been set.
1139 C<$@> is set if the string to be C<eval>-ed did not compile (this
1140 may happen if C<open> or C<close> were imported with bad prototypes),
1141 or if Perl code executed during evaluation die()d . In these cases
1142 the value of $@ is the compile error, or the argument to C<die>
1143 (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1146 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1147 and C<close> are translated to calls in the C run-time library and
1148 thence to the operating system kernel. C<$!> is set to the C library's
1149 C<errno> if one of these calls fails.
1151 Under a few operating systems, C<$^E> may contain a more verbose
1152 error indicator, such as in this case, "CDROM tray not closed."
1153 Systems that do not support extended error messages leave C<$^E>
1156 Finally, C<$?> may be set to non-0 value if the external program
1157 F</cdrom/install> fails. The upper eight bits reflect specific
1158 error conditions encountered by the program (the program's exit()
1159 value). The lower eight bits reflect mode of failure, like signal
1160 death and core dump information See wait(2) for details. In
1161 contrast to C<$!> and C<$^E>, which are set only if error condition
1162 is detected, the variable C<$?> is set on each C<wait> or pipe
1163 C<close>, overwriting the old value. This is more like C<$@>, which
1164 on every eval() is always set on failure and cleared on success.
1166 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1169 =head2 Technical Note on the Syntax of Variable Names
1171 Variable names in Perl can have several formats. Usually, they
1172 must begin with a letter or underscore, in which case they can be
1173 arbitrarily long (up to an internal limit of 251 characters) and
1174 may contain letters, digits, underscores, or the special sequence
1175 C<::> or C<'>. In this case, the part before the last C<::> or
1176 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1178 Perl variable names may also be a sequence of digits or a single
1179 punctuation or control character. These names are all reserved for
1180 special uses by Perl; for example, the all-digits names are used
1181 to hold data captured by backreferences after a regular expression
1182 match. Perl has a special syntax for the single-control-character
1183 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1184 character. For example, the notation C<$^W> (dollar-sign caret
1185 C<W>) is the scalar variable whose name is the single character
1186 control-C<W>. This is better than typing a literal control-C<W>
1189 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1190 strings that begin with control characters (or better yet, a caret).
1191 These variables must be written in the form C<${^Foo}>; the braces
1192 are not optional. C<${^Foo}> denotes the scalar variable whose
1193 name is a control-C<F> followed by two C<o>'s. These variables are
1194 reserved for future special uses by Perl, except for the ones that
1195 begin with C<^_> (control-underscore or caret-underscore). No
1196 control-character name that begins with C<^_> will acquire a special
1197 meaning in any future version of Perl; such names may therefore be
1198 used safely in programs. C<$^_> itself, however, I<is> reserved.
1200 Perl identifiers that begin with digits, control characters, or
1201 punctuation characters are exempt from the effects of the C<package>
1202 declaration and are always forced to be in package C<main>. A few
1203 other names are also exempt:
1211 In particular, the new special C<${^_XYZ}> variables are always taken
1212 to be in package C<main>, regardless of any C<package> declarations
1217 Due to an unfortunate accident of Perl's implementation, C<use
1218 English> imposes a considerable performance penalty on all regular
1219 expression matches in a program, regardless of whether they occur
1220 in the scope of C<use English>. For that reason, saying C<use
1221 English> in libraries is strongly discouraged. See the
1222 Devel::SawAmpersand module documentation from CPAN
1223 (http://www.perl.com/CPAN/modules/by-module/Devel/)
1224 for more information.
1226 Having to even think about the C<$^S> variable in your exception
1227 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1228 invites grievous and difficult to track down errors. Avoid it
1229 and use an C<END{}> or CORE::GLOBAL::die override instead.