Programming Perl, 4th Edition

The Default Package

The default package is main, just like the top-level subroutine name in C. Unless you say otherwise (coming up), all variables are in this package. These are the same:

#!/usr/bin/perl

$name       = 'Amelia';
$main::name = 'Amelia';

$type       = 'Camel';
$main::type = 'Camel';

Under strict, you have to say otherwise because that pragma doesn’t let you use undeclared variables:

#!/usr/bin/perl

use v5.12;

$name       = 'Amelia';  # compile–time error
$main::name = 'Amelia';

$type       = 'Camel';   # compile–time error
$main::type = 'Camel';

Only identifiers (names starting with letters or an underscore) are stored in a package’s symbol table. All other symbols are kept in the main package, including the nonalphabetic variables, like $!, $?, and $_.^[139] In addition, when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC, and SIG are forced to be in package main, even when used for other purposes than their built-in ones. Don’t name your package m, s, y, tr, q, qq, qr, qw, or qx unless you’re looking for a lot of trouble. For instance, you won’t be able to use the qualified form of an identifier as a filehandle because it will be interpreted instead as a pattern match, a substitution, or a transliteration.

^[139] You can have a lexical $_ in v5.10, though.