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 $MULTILINE_MATCHING
196 Set to a non-zero integer value to do multi-line matching within a
197 string, 0 (or undefined) to tell Perl that it can assume that strings
198 contain a single line, for the purpose of optimizing pattern matches.
199 Pattern matches on strings containing multiple newlines can produce
200 confusing results when C<$*> is 0 or undefined. Default is undefined.
201 (Mnemonic: * matches multiple things.) This variable influences the
202 interpretation of only C<^> and C<$>. A literal newline can be searched
203 for even when C<$* == 0>.
205 Use of C<$*> is deprecated in modern Perl, supplanted by
206 the C</s> and C</m> modifiers on pattern matching.
208 Assigning a non-numerical value to C<$*> triggers a warning (and makes
209 C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
210 makes that an implicit C<int> is applied on the value.
212 =item input_line_number HANDLE EXPR
214 =item $INPUT_LINE_NUMBER
220 The current input record number for the last file handle from which
221 you just read() (or called a C<seek> or C<tell> on). The value
222 may be different from the actual physical line number in the file,
223 depending on what notion of "line" is in effect--see C<$/> on how
224 to change that. An explicit close on a filehandle resets the line
225 number. Because C<< <> >> never does an explicit close, line
226 numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
227 Consider this variable read-only: setting it does not reposition
228 the seek pointer; you'll have to do that on your own. Localizing C<$.>
229 has the effect of also localizing Perl's notion of "the last read
230 filehandle". (Mnemonic: many programs use "." to mean the current line
233 =item input_record_separator HANDLE EXPR
235 =item $INPUT_RECORD_SEPARATOR
241 The input record separator, newline by default. This
242 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
243 variable, including treating empty lines as a terminator if set to
244 the null string. (An empty line cannot contain any spaces
245 or tabs.) You may set it to a multi-character string to match a
246 multi-character terminator, or to C<undef> to read through the end
247 of file. Setting it to C<"\n\n"> means something slightly
248 different than setting to C<"">, if the file contains consecutive
249 empty lines. Setting to C<""> will treat two or more consecutive
250 empty lines as a single empty line. Setting to C<"\n\n"> will
251 blindly assume that the next input character belongs to the next
252 paragraph, even if it's a newline. (Mnemonic: / delimits
253 line boundaries when quoting poetry.)
255 undef $/; # enable "slurp" mode
256 $_ = <FH>; # whole file now here
259 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
260 better for something. :-)
262 Setting C<$/> to a reference to an integer, scalar containing an integer, or
263 scalar that's convertible to an integer will attempt to read records
264 instead of lines, with the maximum record size being the referenced
267 $/ = \32768; # or \"32768", or \$var_containing_32768
271 will read a record of no more than 32768 bytes from FILE. If you're
272 not reading from a record-oriented file (or your OS doesn't have
273 record-oriented files), then you'll likely get a full chunk of data
274 with every read. If a record is larger than the record size you've
275 set, you'll get the record back in pieces.
277 On VMS, record reads are done with the equivalent of C<sysread>,
278 so it's best not to mix record and non-record reads on the same
279 file. (This is unlikely to be a problem, because any file you'd
280 want to read in record mode is probably unusable in line mode.)
281 Non-VMS systems do normal I/O, so it's safe to mix record and
282 non-record reads of a file.
284 See also L<perlport/"Newlines">. Also see C<$.>.
286 =item autoflush HANDLE EXPR
288 =item $OUTPUT_AUTOFLUSH
292 If set to nonzero, forces a flush right away and after every write
293 or print on the currently selected output channel. Default is 0
294 (regardless of whether the channel is really buffered by the
295 system or not; C<$|> tells you only whether you've asked Perl
296 explicitly to flush after each write). STDOUT will
297 typically be line buffered if output is to the terminal and block
298 buffered otherwise. Setting this variable is useful primarily when
299 you are outputting to a pipe or socket, such as when you are running
300 a Perl program under B<rsh> and want to see the output as it's
301 happening. This has no effect on input buffering. See L<perlfunc/getc>
302 for that. (Mnemonic: when you want your pipes to be piping hot.)
304 =item output_field_separator HANDLE EXPR
306 =item $OUTPUT_FIELD_SEPARATOR
312 The output field separator for the print operator. Ordinarily the
313 print operator simply prints out its arguments without further
314 adornment. To get behavior more like B<awk>, set this variable as
315 you would set B<awk>'s OFS variable to specify what is printed
316 between fields. (Mnemonic: what is printed when there is a "," in
317 your print statement.)
319 =item output_record_separator HANDLE EXPR
321 =item $OUTPUT_RECORD_SEPARATOR
327 The output record separator for the print operator. Ordinarily the
328 print operator simply prints out its arguments as is, with no
329 trailing newline or other end-of-record string added. To get
330 behavior more like B<awk>, set this variable as you would set
331 B<awk>'s ORS variable to specify what is printed at the end of the
332 print. (Mnemonic: you set C<$\> instead of adding "\n" at the
333 end of the print. Also, it's just like C<$/>, but it's what you
334 get "back" from Perl.)
336 =item $LIST_SEPARATOR
340 This is like C<$,> except that it applies to array and slice values
341 interpolated into a double-quoted string (or similar interpreted
342 string). Default is a space. (Mnemonic: obvious, I think.)
344 =item $SUBSCRIPT_SEPARATOR
350 The subscript separator for multidimensional array emulation. If you
351 refer to a hash element as
357 $foo{join($;, $a, $b, $c)}
361 @foo{$a,$b,$c} # a slice--note the @
365 ($foo{$a},$foo{$b},$foo{$c})
367 Default is "\034", the same as SUBSEP in B<awk>. If your
368 keys contain binary data there might not be any safe value for C<$;>.
369 (Mnemonic: comma (the syntactic subscript separator) is a
370 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
371 taken for something more important.)
373 Consider using "real" multidimensional arrays as described
380 The output format for printed numbers. This variable is a half-hearted
381 attempt to emulate B<awk>'s OFMT variable. There are times, however,
382 when B<awk> and Perl have differing notions of what counts as
383 numeric. The initial value is "%.I<n>g", where I<n> is the value
384 of the macro DBL_DIG from your system's F<float.h>. This is different from
385 B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
386 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
388 Use of C<$#> is deprecated.
390 =item format_page_number HANDLE EXPR
392 =item $FORMAT_PAGE_NUMBER
396 The current page number of the currently selected output channel.
398 (Mnemonic: % is page number in B<nroff>.)
400 =item format_lines_per_page HANDLE EXPR
402 =item $FORMAT_LINES_PER_PAGE
406 The current page length (printable lines) of the currently selected
407 output channel. Default is 60.
409 (Mnemonic: = has horizontal lines.)
411 =item format_lines_left HANDLE EXPR
413 =item $FORMAT_LINES_LEFT
417 The number of lines left on the page of the currently selected output
420 (Mnemonic: lines_on_page - lines_printed.)
422 =item @LAST_MATCH_START
426 $-[0] is the offset of the start of the last successful match.
427 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
428 I<n>-th subpattern, or undef if the subpattern did not match.
430 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
431 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
432 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
433 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
434 matched subgroup in the last successful match. Contrast with
435 C<$#+>, the number of subgroups in the regular expression. Compare
438 This array holds the offsets of the beginnings of the last
439 successful submatches in the currently active dynamic scope.
440 C<$-[0]> is the offset into the string of the beginning of the
441 entire match. The I<n>th element of this array holds the offset
442 of the I<n>th submatch, so C<$+[1]> is the offset where $1
443 begins, C<$+[2]> the offset where $2 begins, and so on.
444 You can use C<$#-> to determine how many subgroups were in the
445 last successful match. Compare with the C<@+> variable.
447 After a match against some variable $var:
451 =item C<$`> is the same as C<substr($var, 0, $-[0])>
453 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
455 =item C<$'> is the same as C<substr($var, $+[0])>
457 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
459 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
461 =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
465 =item format_name HANDLE EXPR
471 The name of the current report format for the currently selected output
472 channel. Default is the name of the filehandle. (Mnemonic: brother to
475 =item format_top_name HANDLE EXPR
477 =item $FORMAT_TOP_NAME
481 The name of the current top-of-page format for the currently selected
482 output channel. Default is the name of the filehandle with _TOP
483 appended. (Mnemonic: points to top of page.)
485 =item format_line_break_characters HANDLE EXPR
487 =item $FORMAT_LINE_BREAK_CHARACTERS
491 The current set of characters after which a string may be broken to
492 fill continuation fields (starting with ^) in a format. Default is
493 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
494 poetry is a part of a line.)
496 =item format_formfeed HANDLE EXPR
498 =item $FORMAT_FORMFEED
502 What formats output as a form feed. Default is \f.
508 The current value of the write() accumulator for format() lines. A format
509 contains formline() calls that put their result into C<$^A>. After
510 calling its format, write() prints out the contents of C<$^A> and empties.
511 So you never really see the contents of C<$^A> unless you call
512 formline() yourself and then look at it. See L<perlform> and
513 L<perlfunc/formline()>.
519 The status returned by the last pipe close, backtick (C<``>) command,
520 successful call to wait() or waitpid(), or from the system()
521 operator. This is just the 16-bit status word returned by the
522 wait() system call (or else is made up to look like it). Thus, the
523 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
524 C<$? & 127> gives which signal, if any, the process died from, and
525 C<$? & 128> reports whether there was a core dump. (Mnemonic:
526 similar to B<sh> and B<ksh>.)
528 Additionally, if the C<h_errno> variable is supported in C, its value
529 is returned via $? if any C<gethost*()> function fails.
531 If you have installed a signal handler for C<SIGCHLD>, the
532 value of C<$?> will usually be wrong outside that handler.
534 Inside an C<END> subroutine C<$?> contains the value that is going to be
535 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
536 change the exit status of your program. For example:
539 $? = 1 if $? == 255; # die would make it 255
542 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
543 actual VMS exit status, instead of the default emulation of POSIX
546 Also see L<Error Indicators>.
554 If used numerically, yields the current value of the C C<errno>
555 variable, with all the usual caveats. (This means that you shouldn't
556 depend on the value of C<$!> to be anything in particular unless
557 you've gotten a specific error return indicating a system error.)
558 If used an a string, yields the corresponding system error string.
559 You can assign a number to C<$!> to set I<errno> if, for instance,
560 you want C<"$!"> to return the string for error I<n>, or you want
561 to set the exit value for the die() operator. (Mnemonic: What just
564 Also see L<Error Indicators>.
566 =item $EXTENDED_OS_ERROR
570 Error information specific to the current operating system. At
571 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
572 (and for MacPerl). On all other platforms, C<$^E> is always just
575 Under VMS, C<$^E> provides the VMS status value from the last
576 system error. This is more specific information about the last
577 system error than that provided by C<$!>. This is particularly
578 important when C<$!> is set to B<EVMSERR>.
580 Under OS/2, C<$^E> is set to the error code of the last call to
581 OS/2 API either via CRT, or directly from perl.
583 Under Win32, C<$^E> always returns the last error information
584 reported by the Win32 call C<GetLastError()> which describes
585 the last error from within the Win32 API. Most Win32-specific
586 code will report errors via C<$^E>. ANSI C and Unix-like calls
587 set C<errno> and so most portable Perl code will report errors
590 Caveats mentioned in the description of C<$!> generally apply to
591 C<$^E>, also. (Mnemonic: Extra error explanation.)
593 Also see L<Error Indicators>.
599 The Perl syntax error message from the last eval() operator. If null, the
600 last eval() parsed and executed correctly (although the operations you
601 invoked may have failed in the normal fashion). (Mnemonic: Where was
602 the syntax error "at"?)
604 Warning messages are not collected in this variable. You can,
605 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
608 Also see L<Error Indicators>.
616 The process number of the Perl running this script. You should
617 consider this variable read-only, although it will be altered
618 across fork() calls. (Mnemonic: same as shells.)
626 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
627 if you're running setuid.) You can change both the real uid and
628 the effective uid at the same time by using POSIX::setuid().
630 =item $EFFECTIVE_USER_ID
636 The effective uid of this process. Example:
638 $< = $>; # set real to effective uid
639 ($<,$>) = ($>,$<); # swap real and effective uid
641 You can change both the effective uid and the real uid at the same
642 time by using POSIX::setuid().
644 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
645 C<< $< >> and C<< $> >> can be swapped only on machines
646 supporting setreuid().
654 The real gid of this process. If you are on a machine that supports
655 membership in multiple groups simultaneously, gives a space separated
656 list of groups you are in. The first number is the one returned by
657 getgid(), and the subsequent ones by getgroups(), one of which may be
658 the same as the first number.
660 However, a value assigned to C<$(> must be a single number used to
661 set the real gid. So the value given by C<$(> should I<not> be assigned
662 back to C<$(> without being forced numeric, such as by adding zero.
664 You can change both the real gid and the effective gid at the same
665 time by using POSIX::setgid().
667 (Mnemonic: parentheses are used to I<group> things. The real gid is the
668 group you I<left>, if you're running setgid.)
670 =item $EFFECTIVE_GROUP_ID
676 The effective gid of this process. If you are on a machine that
677 supports membership in multiple groups simultaneously, gives a space
678 separated list of groups you are in. The first number is the one
679 returned by getegid(), and the subsequent ones by getgroups(), one of
680 which may be the same as the first number.
682 Similarly, a value assigned to C<$)> must also be a space-separated
683 list of numbers. The first number sets the effective gid, and
684 the rest (if any) are passed to setgroups(). To get the effect of an
685 empty list for setgroups(), just repeat the new effective gid; that is,
686 to force an effective gid of 5 and an effectively empty setgroups()
687 list, say C< $) = "5 5" >.
689 You can change both the effective gid and the real gid at the same
690 time by using POSIX::setgid() (use only a single numeric argument).
692 (Mnemonic: parentheses are used to I<group> things. The effective gid
693 is the group that's I<right> for you, if you're running setgid.)
695 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
696 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
697 and C<$)> can be swapped only on machines supporting setregid().
703 Contains the name of the program being executed. On some operating
704 systems assigning to C<$0> modifies the argument area that the B<ps>
705 program sees. This is more useful as a way of indicating the current
706 program state than it is for hiding the program you're running.
707 (Mnemonic: same as B<sh> and B<ksh>.)
709 Note for BSD users: setting C<$0> does not completely remove "perl"
710 from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
711 result in C<"perl: foobar (perl)">. This is an operating system
716 The index of the first element in an array, and of the first character
717 in a substring. Default is 0, but you could theoretically set it
718 to 1 to make Perl behave more like B<awk> (or Fortran) when
719 subscripting and when evaluating the index() and substr() functions.
720 (Mnemonic: [ begins subscripts.)
722 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
723 directive, and cannot influence the behavior of any other file.
724 Its use is highly discouraged.
728 The version + patchlevel / 1000 of the Perl interpreter. This variable
729 can be used to determine whether the Perl interpreter executing a
730 script is in the right range of versions. (Mnemonic: Is this version
731 of perl in the right bracket?) Example:
733 warn "No checksumming!\n" if $] < 3.019;
735 See also the documentation of C<use VERSION> and C<require VERSION>
736 for a convenient way to fail if the running Perl interpreter is too old.
738 The use of this variable is deprecated. The floating point representation
739 can sometimes lead to inaccurate numeric comparisons. See C<$^V> for a
740 more modern representation of the Perl version that allows accurate string
747 The current value of the flag associated with the B<-c> switch.
748 Mainly of use with B<-MO=...> to allow code to alter its behavior
749 when being compiled, such as for example to AUTOLOAD at compile
750 time rather than normal, deferred loading. See L<perlcc>. Setting
751 C<$^C = 1> is similar to calling C<B::minus_c>.
757 The current value of the debugging flags. (Mnemonic: value of B<-D>
764 The maximum system file descriptor, ordinarily 2. System file
765 descriptors are passed to exec()ed processes, while higher file
766 descriptors are not. Also, during an open(), system file descriptors are
767 preserved even if the open() fails. (Ordinary file descriptors are
768 closed before the open() is attempted.) The close-on-exec
769 status of a file descriptor will be decided according to the value of
770 C<$^F> when the corresponding file, pipe, or socket was opened, not the
775 WARNING: This variable is strictly for internal use only. Its availability,
776 behavior, and contents are subject to change without notice.
778 This variable contains compile-time hints for the Perl interpreter. At the
779 end of compilation of a BLOCK the value of this variable is restored to the
780 value when the interpreter started to compile the BLOCK.
782 When perl begins to parse any block construct that provides a lexical scope
783 (e.g., eval body, required file, subroutine body, loop body, or conditional
784 block), the existing value of $^H is saved, but its value is left unchanged.
785 When the compilation of the block is completed, it regains the saved value.
786 Between the points where its value is saved and restored, code that
787 executes within BEGIN blocks is free to change the value of $^H.
789 This behavior provides the semantic of lexical scoping, and is used in,
790 for instance, the C<use strict> pragma.
792 The contents should be an integer; different bits of it are used for
793 different pragmatic flags. Here's an example:
795 sub add_100 { $^H |= 0x100 }
802 Consider what happens during execution of the BEGIN block. At this point
803 the BEGIN block has already been compiled, but the body of foo() is still
804 being compiled. The new value of $^H will therefore be visible only while
805 the body of foo() is being compiled.
807 Substitution of the above BEGIN block with:
809 BEGIN { require strict; strict->import('vars') }
811 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
812 version of the same lexical pragma:
814 BEGIN { require strict; strict->import('vars') if $condition }
818 WARNING: This variable is strictly for internal use only. Its availability,
819 behavior, and contents are subject to change without notice.
821 The %^H hash provides the same scoping semantic as $^H. This makes it
822 useful for implementation of lexically scoped pragmas.
828 The current value of the inplace-edit extension. Use C<undef> to disable
829 inplace editing. (Mnemonic: value of B<-i> switch.)
833 By default, running out of memory is an untrappable, fatal error.
834 However, if suitably built, Perl can use the contents of C<$^M>
835 as an emergency memory pool after die()ing. Suppose that your Perl
836 were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
839 $^M = 'a' x (1 << 16);
841 would allocate a 64K buffer for use in an emergency. See the
842 F<INSTALL> file in the Perl distribution for information on how to
843 enable this option. To discourage casual use of this advanced
844 feature, there is no L<English|English> long name for this variable.
850 The name of the operating system under which this copy of Perl was
851 built, as determined during the configuration process. The value
852 is identical to C<$Config{'osname'}>. See also L<Config> and the
853 B<-V> command-line switch documented in L<perlrun>.
859 The internal variable for debugging support. The meanings of the
860 various bits are subject to change, but currently indicate:
866 Debug subroutine enter/exit.
870 Line-by-line debugging.
874 Switch off optimizations.
878 Preserve more data for future interactive inspections.
882 Keep info about source lines on which a subroutine is defined.
886 Start with single-step on.
890 Use subroutine address instead of name when reporting.
894 Report C<goto &subroutine> as well.
898 Provide informative "file" names for evals based on the place they were compiled.
902 Provide informative names to anonymous subroutines based on the place they
907 Some bits may be relevant at compile-time only, some at
908 run-time only. This is a new mechanism and the details may change.
910 =item $LAST_REGEXP_CODE_RESULT
914 The result of evaluation of the last successful C<(?{ code })>
915 regular expression assertion (see L<perlre>). May be written to.
917 =item $EXCEPTIONS_BEING_CAUGHT
921 Current state of the interpreter. Undefined if parsing of the current
922 module/eval is not finished (may happen in $SIG{__DIE__} and
923 $SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
929 The time at which the program began running, in seconds since the
930 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
931 and B<-C> filetests are based on this value.
937 The revision, version, and subversion of the Perl interpreter, represented
938 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
939 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
940 C<$^V eq v5.6.0>. Note that the characters in this string value can
941 potentially be in Unicode range.
943 This can be used to determine whether the Perl interpreter executing a
944 script is in the right range of versions. (Mnemonic: use ^V for Version
947 warn "No "our" declarations!\n" if $^V and $^V lt v5.6.0;
949 See the documentation of C<use VERSION> and C<require VERSION>
950 for a convenient way to fail if the running Perl interpreter is too old.
952 See also C<$]> for an older representation of the Perl version.
958 The current value of the warning switch, initially true if B<-w>
959 was used, false otherwise, but directly modifiable. (Mnemonic:
960 related to the B<-w> switch.) See also L<warnings>.
962 =item ${^WARNING_BITS}
964 The current set of warning checks enabled by the C<use warnings> pragma.
965 See the documentation of C<warnings> for more details.
967 =item ${^WIDE_SYSTEM_CALLS}
969 Global flag that enables system calls made by Perl to use wide character
970 APIs native to the system, if available. This is currently only implemented
971 on the Windows platform.
973 This can also be enabled from the command line using the C<-C> switch.
975 The initial value is typically C<0> for compatibility with Perl versions
976 earlier than 5.6, but may be automatically set to C<1> by Perl if the system
977 provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
979 The C<bytes> pragma always overrides the effect of this flag in the current
980 lexical scope. See L<bytes>.
982 =item $EXECUTABLE_NAME
986 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
987 This may not be a full pathname, nor even necessarily in your path.
991 contains the name of the current file when reading from <>.
995 The array @ARGV contains the command-line arguments intended for
996 the script. C<$#ARGV> is generally the number of arguments minus
997 one, because C<$ARGV[0]> is the first argument, I<not> the program's
998 command name itself. See C<$0> for the command name.
1002 The array @INC contains the list of places that the C<do EXPR>,
1003 C<require>, or C<use> constructs look for their library files. It
1004 initially consists of the arguments to any B<-I> command-line
1005 switches, followed by the default Perl library, probably
1006 F</usr/local/lib/perl>, followed by ".", to represent the current
1007 directory. If you need to modify this at runtime, you should use
1008 the C<use lib> pragma to get the machine-dependent library properly
1011 use lib '/mypath/libdir/';
1016 Within a subroutine the array @_ contains the parameters passed to that
1017 subroutine. See L<perlsub>.
1021 The hash %INC contains entries for each filename included via the
1022 C<do>, C<require>, or C<use> operators. The key is the filename
1023 you specified (with module names converted to pathnames), and the
1024 value is the location of the file found. The C<require>
1025 operator uses this hash to determine whether a particular file has
1026 already been included.
1032 The hash %ENV contains your current environment. Setting a
1033 value in C<ENV> changes the environment for any child processes
1034 you subsequently fork() off.
1040 The hash %SIG contains signal handlers for signals. For example:
1042 sub handler { # 1st argument is signal name
1044 print "Caught a SIG$sig--shutting down\n";
1049 $SIG{'INT'} = \&handler;
1050 $SIG{'QUIT'} = \&handler;
1052 $SIG{'INT'} = 'DEFAULT'; # restore default action
1053 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1055 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1056 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1059 Here are some other examples:
1061 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1062 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1063 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1064 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1066 Be sure not to use a bareword as the name of a signal handler,
1067 lest you inadvertently call it.
1069 If your system has the sigaction() function then signal handlers are
1070 installed using it. This means you get reliable signal handling. If
1071 your system has the SA_RESTART flag it is used when signals handlers are
1072 installed. This means that system calls for which restarting is supported
1073 continue rather than returning when a signal arrives. If you want your
1074 system calls to be interrupted by signal delivery then do something like
1077 use POSIX ':signal_h';
1080 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1081 or die "Error setting SIGALRM handler: $!\n";
1085 Certain internal hooks can be also set using the %SIG hash. The
1086 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1087 about to be printed. The warning message is passed as the first
1088 argument. The presence of a __WARN__ hook causes the ordinary printing
1089 of warnings to STDERR to be suppressed. You can use this to save warnings
1090 in a variable, or turn warnings into fatal errors, like this:
1092 local $SIG{__WARN__} = sub { die $_[0] };
1095 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1096 is about to be thrown. The error message is passed as the first
1097 argument. When a __DIE__ hook routine returns, the exception
1098 processing continues as it would have in the absence of the hook,
1099 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
1100 The C<__DIE__> handler is explicitly disabled during the call, so that you
1101 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1103 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1104 even inside an eval(). Do not use this to rewrite a pending exception
1105 in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1106 This strange action at a distance may be fixed in a future release
1107 so that C<$SIG{__DIE__}> is only called if your program is about
1108 to exit, as was the original intent. Any other use is deprecated.
1110 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1111 they may be called to report (probable) errors found by the parser.
1112 In such a case the parser may be in inconsistent state, so any
1113 attempt to evaluate Perl code from such a handler will probably
1114 result in a segfault. This means that warnings or errors that
1115 result from parsing Perl should be used with extreme caution, like
1118 require Carp if defined $^S;
1119 Carp::confess("Something wrong") if defined &Carp::confess;
1120 die "Something wrong, but could not load Carp to give backtrace...
1121 To see backtrace try starting Perl with -MCarp switch";
1123 Here the first line will load Carp I<unless> it is the parser who
1124 called the handler. The second line will print backtrace and die if
1125 Carp was available. The third line will be executed only if Carp was
1128 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1129 L<warnings> for additional information.
1133 =head2 Error Indicators
1135 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1136 about different types of error conditions that may appear during
1137 execution of a Perl program. The variables are shown ordered by
1138 the "distance" between the subsystem which reported the error and
1139 the Perl process. They correspond to errors detected by the Perl
1140 interpreter, C library, operating system, or an external program,
1143 To illustrate the differences between these variables, consider the
1144 following Perl expression, which uses a single-quoted string:
1147 open PIPE, "/cdrom/install |";
1149 close PIPE or die "bad pipe: $?, $!";
1152 After execution of this statement all 4 variables may have been set.
1154 C<$@> is set if the string to be C<eval>-ed did not compile (this
1155 may happen if C<open> or C<close> were imported with bad prototypes),
1156 or if Perl code executed during evaluation die()d . In these cases
1157 the value of $@ is the compile error, or the argument to C<die>
1158 (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1161 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1162 and C<close> are translated to calls in the C run-time library and
1163 thence to the operating system kernel. C<$!> is set to the C library's
1164 C<errno> if one of these calls fails.
1166 Under a few operating systems, C<$^E> may contain a more verbose
1167 error indicator, such as in this case, "CDROM tray not closed."
1168 Systems that do not support extended error messages leave C<$^E>
1171 Finally, C<$?> may be set to non-0 value if the external program
1172 F</cdrom/install> fails. The upper eight bits reflect specific
1173 error conditions encountered by the program (the program's exit()
1174 value). The lower eight bits reflect mode of failure, like signal
1175 death and core dump information See wait(2) for details. In
1176 contrast to C<$!> and C<$^E>, which are set only if error condition
1177 is detected, the variable C<$?> is set on each C<wait> or pipe
1178 C<close>, overwriting the old value. This is more like C<$@>, which
1179 on every eval() is always set on failure and cleared on success.
1181 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1184 =head2 Technical Note on the Syntax of Variable Names
1186 Variable names in Perl can have several formats. Usually, they
1187 must begin with a letter or underscore, in which case they can be
1188 arbitrarily long (up to an internal limit of 251 characters) and
1189 may contain letters, digits, underscores, or the special sequence
1190 C<::> or C<'>. In this case, the part before the last C<::> or
1191 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1193 Perl variable names may also be a sequence of digits or a single
1194 punctuation or control character. These names are all reserved for
1195 special uses by Perl; for example, the all-digits names are used
1196 to hold data captured by backreferences after a regular expression
1197 match. Perl has a special syntax for the single-control-character
1198 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1199 character. For example, the notation C<$^W> (dollar-sign caret
1200 C<W>) is the scalar variable whose name is the single character
1201 control-C<W>. This is better than typing a literal control-C<W>
1204 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1205 strings that begin with control characters (or better yet, a caret).
1206 These variables must be written in the form C<${^Foo}>; the braces
1207 are not optional. C<${^Foo}> denotes the scalar variable whose
1208 name is a control-C<F> followed by two C<o>'s. These variables are
1209 reserved for future special uses by Perl, except for the ones that
1210 begin with C<^_> (control-underscore or caret-underscore). No
1211 control-character name that begins with C<^_> will acquire a special
1212 meaning in any future version of Perl; such names may therefore be
1213 used safely in programs. C<$^_> itself, however, I<is> reserved.
1215 Perl identifiers that begin with digits, control characters, or
1216 punctuation characters are exempt from the effects of the C<package>
1217 declaration and are always forced to be in package C<main>. A few
1218 other names are also exempt:
1226 In particular, the new special C<${^_XYZ}> variables are always taken
1227 to be in package C<main>, regardless of any C<package> declarations
1232 Due to an unfortunate accident of Perl's implementation, C<use
1233 English> imposes a considerable performance penalty on all regular
1234 expression matches in a program, regardless of whether they occur
1235 in the scope of C<use English>. For that reason, saying C<use
1236 English> in libraries is strongly discouraged. See the
1237 Devel::SawAmpersand module documentation from CPAN
1238 (http://www.perl.com/CPAN/modules/by-module/Devel/)
1239 for more information.
1241 Having to even think about the C<$^S> variable in your exception
1242 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1243 invites grievous and difficult to track down errors. Avoid it
1244 and use an C<END{}> or CORE::GLOBAL::die override instead.