One of the weaknesses of Perl's OO implementation is that you have to know a class's internal representation in order to inherit from it.

Actually, this is not strictly true. He could use an inside-out-object-like design, and avoid using the object reference to store the data at all. He could initialize the object using the original constructor, and rebless it into his package. Voila, inheritance without knowing about the parent's internal representation. By the way, I am not saying this is the best design in these circumstances, only that it is possible.