perlvar.1

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.\" ========================================================================
.\"
.IX Title "PERLVAR 1"
.TH PERLVAR 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
perlvar \- Perl predefined variables
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
.Sh "Predefined Names"
.IX Subsection "Predefined Names"
The following names have special meaning to Perl.  Most 
punctuation names have reasonable mnemonics, or analogs in the
shells.  Nevertheless, if you wish to use long variable names,
you need only say
.PP
.Vb 1
\&    use English;
.Ve
.PP
at the top of your program. This aliases all the short names to the long
names in the current package. Some even have medium names, generally
borrowed from \fBawk\fR. In general, it's best to use the
.PP
.Vb 1
\&    use English \*(Aq\-no_match_vars\*(Aq;
.Ve
.PP
invocation if you don't need \f(CW$PREMATCH\fR, \f(CW$MATCH\fR, or \f(CW$POSTMATCH\fR, as it avoids
a certain performance hit with the use of regular expressions. See
English.
.PP
Variables that depend on the currently selected filehandle may be set by
calling an appropriate object method on the IO::Handle object, although
this is less efficient than using the regular built-in variables. (Summary
lines below for this contain the word \s-1HANDLE\s0.) First you must say
.PP
.Vb 1
\&    use IO::Handle;
.Ve
.PP
after which you may use either
.PP
.Vb 1
\&    method HANDLE EXPR
.Ve
.PP
or more safely,
.PP
.Vb 1
\&    HANDLE\->method(EXPR)
.Ve
.PP
Each method returns the old value of the IO::Handle attribute.
The methods each take an optional \s-1EXPR\s0, which, if supplied, specifies the
new value for the IO::Handle attribute in question.  If not supplied,
most methods do nothing to the current value\*(--except for
\&\fIautoflush()\fR, which will assume a 1 for you, just to be different.
.PP
Because loading in the IO::Handle class is an expensive operation, you should
learn how to use the regular built-in variables.
.PP
A few of these variables are considered \*(L"read-only\*(R".  This means that if
you try to assign to this variable, either directly or indirectly through
a reference, you'll raise a run-time exception.
.PP
You should be very careful when modifying the default values of most
special variables described in this document. In most cases you want
to localize these variables before changing them, since if you don't,
the change may affect other modules which rely on the default values
of the special variables that you have changed. This is one of the
correct ways to read the whole file at once:
.PP
.Vb 4
\&    open my $fh, "foo" or die $!;
\&    local $/; # enable localized slurp mode
\&    my $content = <$fh>;
\&    close $fh;
.Ve
.PP
But the following code is quite bad:
.PP
.Vb 4
\&    open my $fh, "foo" or die $!;
\&    undef $/; # enable slurp mode
\&    my $content = <$fh>;
\&    close $fh;
.Ve
.PP
since some other module, may want to read data from some file in the
default \*(L"line mode\*(R", so if the code we have just presented has been
executed, the global value of \f(CW$/\fR is now changed for any other code
running inside the same Perl interpreter.
.PP
Usually when a variable is localized you want to make sure that this
change affects the shortest scope possible. So unless you are already
inside some short \f(CW\*(C`{}\*(C'\fR block, you should create one yourself. For
example:
.PP
.Vb 7
\&    my $content = \*(Aq\*(Aq;
\&    open my $fh, "foo" or die $!;
\&    {
\&        local $/;
\&        $content = <$fh>;
\&    }
\&    close $fh;
.Ve
.PP
Here is an example of how your own code can go broken:
.PP
.Vb 8
\&    for (1..5){
\&        nasty_break();
\&        print "$_ ";
\&    }
\&    sub nasty_break {
\&        $_ = 5;
\&        # do something with $_
\&    }
.Ve
.PP
You probably expect this code to print:
.PP
.Vb 1
\&    1 2 3 4 5
.Ve
.PP
but instead you get:
.PP
.Vb 1
\&    5 5 5 5 5
.Ve
.PP
Why? Because \fInasty_break()\fR modifies \f(CW$_\fR without localizing it
first. The fix is to add \fIlocal()\fR:
.PP
.Vb 1
\&        local $_ = 5;
.Ve
.PP
It's easy to notice the problem in such a short example, but in more
complicated code you are looking for trouble if you don't localize
changes to the special variables.
.PP
The following list is ordered by scalar variables first, then the
arrays, then the hashes.
.ie n .IP "$ARG" 8
.el .IP "\f(CW$ARG\fR" 8
.IX Item "$ARG"
.PD 0
.ie n .IP "$_" 8
.el .IP "\f(CW$_\fR" 8
.IX Xref "$_ $ARG"
.IX Item "$_"
.PD
The default input and pattern-searching space.  The following pairs are
equivalent:
.Sp
.Vb 2
\&    while (<>) {...}    # equivalent only in while!
\&    while (defined($_ = <>)) {...}
\&
\&    /^Subject:/
\&    $_ =~ /^Subject:/
\&
\&    tr/a\-z/A\-Z/
\&    $_ =~ tr/a\-z/A\-Z/
\&
\&    chomp
\&    chomp($_)
.Ve
.Sp
Here are the places where Perl will assume \f(CW$_\fR even if you
don't use it:
.RS 8
.IP "\(bu" 3
Various unary functions, including functions like \fIord()\fR and \fIint()\fR, as well
as the all file tests (\f(CW\*(C`\-f\*(C'\fR, \f(CW\*(C`\-d\*(C'\fR) except for \f(CW\*(C`\-t\*(C'\fR, which defaults to
\&\s-1STDIN\s0.
.IP "\(bu" 3
Various list functions like \fIprint()\fR and \fIunlink()\fR.
.IP "\(bu" 3
The pattern matching operations \f(CW\*(C`m//\*(C'\fR, \f(CW\*(C`s///\*(C'\fR, and \f(CW\*(C`tr///\*(C'\fR when used
without an \f(CW\*(C`=~\*(C'\fR operator.
.IP "\(bu" 3
The default iterator variable in a \f(CW\*(C`foreach\*(C'\fR loop if no other
variable is supplied.
.IP "\(bu" 3
The implicit iterator variable in the \fIgrep()\fR and \fImap()\fR functions.
.IP "\(bu" 3
The default place to put an input record when a \f(CW\*(C`<FH>\*(C'\fR
operation's result is tested by itself as the sole criterion of a \f(CW\*(C`while\*(C'\fR
test.  Outside a \f(CW\*(C`while\*(C'\fR test, this will not happen.
.RE
.RS 8
.Sp
As \f(CW$_\fR is a global variable, this may lead in some cases to unwanted
side-effects.  As of perl 5.9.1, you can now use a lexical version of
\&\f(CW$_\fR by declaring it in a file or in a block with \f(CW\*(C`my\*(C'\fR.  Moreover,
declaring \f(CW\*(C`our $_\*(C'\fR restores the global \f(CW$_\fR in the current scope.
.Sp
(Mnemonic: underline is understood in certain operations.)
.RE
.ie n .IP "$a" 8
.el .IP "\f(CW$a\fR" 8
.IX Item "$a"
.PD 0
.ie n .IP "$b" 8
.el .IP "\f(CW$b\fR" 8
.IX Xref "$a $b"
.IX Item "$b"
.PD
Special package variables when using \fIsort()\fR, see \*(L"sort\*(R" in perlfunc.