=back
+As C<$_> is a global variable, this may lead in some cases to unwanted
+side-effects. As of perl 5.9.1, you can now use a lexical version of
+C<$_> by declaring it in a file or in a block with C<my>. Moreover,
+declaring C<our $> restores the global C<$_> in the current scope.
+
(Mnemonic: underline is understood in certain operations.)
=back
Special package variables when using sort(), see L<perlfunc/sort>.
Because of this specialness $a and $b don't need to be declared
-(using local(), use vars, or our()) even when using the strict
-vars pragma. Don't lexicalize them with C<my $a> or C<my $b>
-if you want to be able to use them in the sort() comparison block
-or function.
+(using use vars, or our()) even when using the C<strict 'vars'> pragma.
+Don't lexicalize them with C<my $a> or C<my $b> if you want to be
+able to use them in the sort() comparison block or function.
=back
how many subgroups were in the last successful match. See the
examples given for the C<@-> variable.
-=item $MULTILINE_MATCHING
-
-=item $*
-
-Set to a non-zero integer value to do multi-line matching within a
-string, 0 (or undefined) to tell Perl that it can assume that strings
-contain a single line, for the purpose of optimizing pattern matches.
-Pattern matches on strings containing multiple newlines can produce
-confusing results when C<$*> is 0 or undefined. Default is undefined.
-(Mnemonic: * matches multiple things.) This variable influences the
-interpretation of only C<^> and C<$>. A literal newline can be searched
-for even when C<$* == 0>.
-
-Use of C<$*> is deprecated in modern Perl, supplanted by
-the C</s> and C</m> modifiers on pattern matching.
-
-Assigning a non-numerical value to C<$*> triggers a warning (and makes
-C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
-makes that an implicit C<int> is applied on the value.
-
=item HANDLE->input_line_number(EXPR)
=item $INPUT_LINE_NUMBER
Consider using "real" multidimensional arrays as described
in L<perllol>.
-=item $OFMT
-
=item $#
The output format for printed numbers. This variable is a half-hearted
C<undef>. A successful system or library call does B<not> set
the variable to zero.
-If used an a string, yields the corresponding system error string.
+If used as a string, yields the corresponding system error string.
You can assign a number to C<$!> to set I<errno> if, for instance,
you want C<"$!"> to return the string for error I<n>, or you want
to set the exit value for the die() operator. (Mnemonic: What just
consider this variable read-only, although it will be altered
across fork() calls. (Mnemonic: same as shells.)
+Note for Linux users: on Linux, the C functions C<getpid()> and
+C<getppid()> return different values from different threads. In order to
+be portable, this behavior is not reflected by C<$$>, whose value remains
+consistent across threads. If you want to call the underlying C<getpid()>,
+you may use the CPAN module C<Linux::Pid>.
+
=item $REAL_USER_ID
=item $UID
=item $0
-Contains the name of the program being executed. On some operating
-systems assigning to C<$0> modifies the argument area that the B<ps>
-program sees. This is more useful as a way of indicating the current
-program state than it is for hiding the program you're running.
-(Mnemonic: same as B<sh> and B<ksh>.)
+Contains the name of the program being executed.
+
+On some (read: not all) operating systems assigning to C<$0> modifies
+the argument area that the C<ps> program sees. On some platforms you
+may have to use special C<ps> options or a different C<ps> to see the
+changes. Modifying the $0 is more useful as a way of indicating the
+current program state than it is for hiding the program you're
+running. (Mnemonic: same as B<sh> and B<ksh>.)
+
+Note that there are platform specific limitations on the the maximum
+length of C<$0>. In the most extreme case it may be limited to the
+space occupied by the original C<$0>.
+
+In some platforms there may be arbitrary amount of padding, for
+example space characters, after the modified name as shown by C<ps>.
+In some platforms this padding may extend all the way to the original
+length of the argument area, no matter what you do (this is the case
+for example with Linux 2.2).
Note for BSD users: setting C<$0> does not completely remove "perl"
-from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
-result in C<"perl: foobar (perl)">. This is an operating system
-feature.
+from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
+result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
+and the " (perl)" suffix are shown depends on your exact BSD variant
+and version). This is an operating system feature, Perl cannot help it.
+
+In multithreaded scripts Perl coordinates the threads so that any
+thread may modify its copy of the C<$0> and the change becomes visible
+to ps(1) (assuming the operating system plays along). Note that the
+the view of C<$0> the other threads have will not change since they
+have their own copies of it.
=item $[
As of release 5 of Perl, assignment to C<$[> is treated as a compiler
directive, and cannot influence the behavior of any other file.
+(That's why you can only assign compile-time constants to it.)
Its use is highly discouraged.
+Note that, unlike other compile-time directives (such as L<strict>),
+assignment to $[ can be seen from outer lexical scopes in the same file.
+However, you can use local() on it to strictly bound its value to a
+lexical block.
+
=item $]
The version + patchlevel / 1000 of the Perl interpreter. This variable
=item $^D
The current value of the debugging flags. (Mnemonic: value of B<-D>
-switch.)
+switch.) May be read or set. Like its command-line equivalent, you can use
+numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
=item $SYSTEM_FD_MAX
is identical to C<$Config{'osname'}>. See also L<Config> and the
B<-V> command-line switch documented in L<perlrun>.
-In Windows platforms the $^O is not very helpful since it is always
-C<MSWin32>, it doesn't tell the difference between 95/98/ME/NT/2000/XP/CE.
-Use Win32::GetOSVersion() (see L<perlport>) and other means (like for
-example the presence/absence of some files) to distinguish between
-the variants.
+In Windows platforms, $^O is not very helpful: since it is always
+C<MSWin32>, it doesn't tell the difference between
+95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
+Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
+between the variants.
=item ${^OPEN}
An internal variable used by PerlIO. A string in two parts, separated
-by a C<\0> byte, the first part is the input disciplines, the second
-part is the output disciplines.
+by a C<\0> byte, the first part describes the input layers, the second
+part describes the output layers.
=item $PERLDB
Provide informative names to anonymous subroutines based on the place they
were compiled.
+=item 0x400
+
+Debug assertion subroutines enter/exit.
+
=back
Some bits may be relevant at compile-time only, some at
=item $^S
-Current state of the interpreter. Undefined if parsing of the current
-module/eval is not finished (may happen in $SIG{__DIE__} and
-$SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
+Current state of the interpreter.
+
+ $^S State
+ --------- -------------------
+ undef Parsing module/eval
+ true (1) Executing an eval
+ false (0) Otherwise
+
+The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
=item $BASETIME
=item ${^TAINT}
-Reflects if taint mode is on or off (i.e. if the program was run with
-B<-T> or not). True for on, false for off.
+Reflects if taint mode is on or off. 1 for on (the program was run with
+B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
+B<-t> or B<-TU>). This variable is read-only.
+
+=item ${^UNICODE}
+
+Reflects certain Unicode settings of Perl. See L<perlrun>
+documentation for the C<-C> switch for more information about
+the possible values. This variable is set during Perl startup
+and is thereafter read-only.
=item $PERL_VERSION
The current set of warning checks enabled by the C<use warnings> pragma.
See the documentation of C<warnings> for more details.
-=item ${^WIDE_SYSTEM_CALLS}
-
-Global flag that enables system calls made by Perl to use wide character
-APIs native to the system, if available. This is currently only implemented
-on the Windows platform.
-
-This can also be enabled from the command line using the C<-C> switch.
-
-The initial value is typically C<0> for compatibility with Perl versions
-earlier than 5.6, but may be automatically set to C<1> by Perl if the system
-provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
-
-The C<bytes> pragma always overrides the effect of this flag in the current
-lexical scope. See L<bytes>.
-
=item $EXECUTABLE_NAME
=item $^X
one, because C<$ARGV[0]> is the first argument, I<not> the program's
command name itself. See C<$0> for the command name.
+=item ARGVOUT
+
+The special filehandle that points to the currently open output file
+when doing edit-in-place processing with B<-i>. Useful when you have
+to do a lot of inserting and don't want to keep modifying $_. See
+L<perlrun> for the B<-i> switch.
+
=item @F
The array @F contains the fields of each line read in when autosplit
lest you inadvertently call it.
If your system has the sigaction() function then signal handlers are
-installed using it. This means you get reliable signal handling. If
-your system has the SA_RESTART flag it is used when signals handlers are
-installed. This means that system calls for which restarting is supported
-continue rather than returning when a signal arrives. If you want your
-system calls to be interrupted by signal delivery then do something like
-this:
-
- use POSIX ':signal_h';
-
- my $alarm = 0;
- sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
- or die "Error setting SIGALRM handler: $!\n";
+installed using it. This means you get reliable signal handling.
-See L<POSIX>.
+The default delivery policy of signals changed in Perl 5.8.0 from
+immediate (also known as "unsafe") to deferred, also known as
+"safe signals". See L<perlipc> for more information.
Certain internal hooks can be also set using the %SIG hash. The
routine indicated by C<$SIG{__WARN__}> is called when a warning message is
Perl identifiers that begin with digits, control characters, or
punctuation characters are exempt from the effects of the C<package>
-declaration and are always forced to be in package C<main>. A few
-other names are also exempt:
+declaration and are always forced to be in package C<main>; they are
+also exempt from C<strict 'vars'> errors. A few other names are also
+exempt in these ways:
ENV STDIN
INC STDOUT
ARGV STDERR
- ARGVOUT
+ ARGVOUT _
SIG
In particular, the new special C<${^_XYZ}> variables are always taken
to be in package C<main>, regardless of any C<package> declarations
-presently in scope.
+presently in scope.
=head1 BUGS
https://perl5.git.perl.org / perl5.git / blobdiff