Ah, sure enough -- if you look at the source of OnlinePayment.pm you'll see that it's got a very specific approach to subclassing already implemented.

It might be straightforward to make a package that was a subclass of a specific processor, such as Unleashed::AuthorizeCard::AuthorizeNet inheriting from both your Unleashed::AuthorizeCard and Business::OnlinePayment::AuthorizeNet.

If you want to dynamically handle whichever processor the end-user selects, you'll probably have to either generate subclasses on the fly, or use delegation rather than inheritance.

(More generally, this illustrates one of the advantages of well-factored frameworks that use delegation internally; for example, I can imagine a version of Business::OnlinePayment that was divided into separate Transaction and Processor classes, which would allow you to subclass Transaction without worrying which Processor it was connecting to.)