# File: main.pl
#! perl
use Modern::Perl;
use Foo;

our $Id = 42;
Foo::foo_id();
say 'In main, $Id is ', $Id;

...

# File: Foo.pm
package Foo;
use Modern::Perl;

sub foo_id
{
    say 'In Foo, $main::Id is ', $main::Id;
    $main::Id = 17;
}

1;