#file Interface1.pm
package Interface1;

sub foo
{
    return "I am one!\n";
}

1;

#file Interface2.pm
package Interface2;

sub foo
{
    return "I am two!\n";
}

1;

#file script.pl
use strict;
use warnings;

use Interface1;
use Interface2;

my $interface;

$interface = "Interface1";
print $interface->foo();

$interface = "Interface2";
print $interface->foo();