Yes, I can see that re-design is not an option for you at the moment, and I agree that modularising and adding use strict will aid both debugging and enhancement.

But why make it all so complicated? You can have lexical variables which are strict-compliant and still package-global — just use our instead of my:

# 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;
[download]

Output:

In Foo, $main::Id is 42
In main, $Id is 17
[download]

You can of course get fancier with @EXPORT, @EXPORT_OK, etc., but using the fully-qualified package names is probably a better strategy as it makes the trail of the global variables self-documenting.

Hope that helps,