I think you must have meant "Bubba->new->abba" produces the error. The below code works as I expect it to, printing 2 then your error, then 1.

#!/usr/bin/perl
use strict;
use warnings;

use Hubba;
use Bubba;

print Hubba->new->abba, "\n";
print Bubba->new->abba, "\n";
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This makes sense to me because if i have code that is using a Hubba, and I make a Bubba, then that same code should be able to use the Bubba as if it is a Hubba, which means the interface shouldn't change. i.e. hubba is always an Int as the base clase says it should be.