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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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10.1 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

use English;

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 awk. In general, it's best to use the

use English '-no_match_vars';

invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids a certain performance hit with the use of regular expressions. See "Use nice English (or awk) names for ugly punctuation variables" (English) in the Perl Library Reference Manual (Volume 2).

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 HANDLE.) First you must say

use IO::Handle;

after which you may use either

method HANDLE EXPR

or more safely,

HANDLE->method(EXPR)

Each method returns the old value of the IO::Handle attribute. The methods each take an optional EXPR, 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 autoflush(), which will assume a 1 for you, just to be different.

Because loading in the IO::Handle class is an expensive operation, you should learn how to use the regular built-in variables.

A few of these variables are considered "read-only". 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.

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:

open my $fh, "<", "foo" or die $!;
local $/; # enable localized slurp mode
my $content = <$fh>;
close $fh;

But the following code is quite bad:

open my $fh, "<", "foo" or die $!;
undef $/; # enable slurp mode
my $content = <$fh>;
close $fh;

since some other module, may want to read data from some file in the default "line mode", so if the code we have just presented has been executed, the global value of $/ is now changed for any other code running inside the same Perl interpreter.

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 {} block, you should create one yourself. For example:

my $content = ”;
open my $fh, "<", "foo" or die $!;
{
    local $/;
    $content = <$fh>;
}
close $fh;

Here is an example of how your own code can go broken:

for (1..5){
    nasty_break();
    print "$_ ";
}
sub nasty_break {
    $_ = 5;
    # do something with $_
}

You probably expect this code to print:

1 2 3 4 5

but instead you get:

5 5 5 5 5

Why? Because nasty_break() modifies $_ without localizing it first. The fix is to add local():

local $_ = 5;

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.

The following list is ordered by scalar variables first, then the arrays, then the hashes.

ISBN 9781906966027Perl Language Reference ManualSee the print edition