Perl truly global variable and tacit global variable.

set_special_globals.pl

#!/usr/bin/perl
# $_ is truly global variable
# $a is especially global varialble (for sort and some function). is not trully?

use strict;
use warnings;
use v5.10;

# initialize strictly
undef $_;
undef $a;
undef $main::_;
undef $main::a;
undef $Foo::_;
undef $Foo::a;

package Foo {
    sub set_a {
        $a = shift;
    }
    sub set_underline {
        $_ = shift;
    }
}

say "a is strictly undefined." if !grep { !defined } ($a, $main::a, $Foo::a);
say "_ is strictly undefined." if !grep { !defined } ($_, $main::_, $Foo::_);

say "set \$a at Foo";
Foo::set_a("foo");
say "a, main::a, Foo::a = " . ( join ",", map { $_ // "(undef)" } $a, $main::a, $Foo::a ) . "";

say "set \$_ at Foo";
Foo::set_underline("foo");
say "_, main::_, Foo::_ = " . ( join ",", map { $_ // "(undef)" } $_, $main::_, $Foo::_ ) . "";

__END__

output:

$ perl set_special_globals.pl
set $a at Foo
a, main::a, Foo::a = (undef),(undef),foo
set $_ at Foo
_, main::_, Foo::_ = foo,foo,(undef)