You can attach it to either a sub reference, or as a scalar to a sub reference

*p2::func = $func;

package p2;

our $x = 100;
func();
[download]

or

*p2::func = \$func;

package p2;

our $x = 100;
$func->();
[download]

However, both of those will still use the '$x' global defined in package 'p1'. To fix that, rely on parameters instead of a global var.

#!/usr/bin/perl

use warnings;
use strict;

package p1;

our $x = 1;
my $func = sub { print $_[0],"\n" };

$func->($x);

*p2::func = $func;

package p2;

our $x = 100;
func($x);
[download]

Or use caller to determine where the function was called from (very messy).

#!/usr/bin/perl

use warnings;
use strict;

package p1;

our $x = 1;
my $func = sub {
    my $pkg = (caller)[0];
    no strict 'refs';
    print ${"${pkg}::x"},"\n"
};

$func->();

*p2::func = $func;

package p2;

our $x = 100;
func();
[download]