While DBIx::Class and Rose::DB::Object are good ORM systems, I think the OP is asking about how to handle inheritance in an ORM system, and it's a question that I often wonder about too. How does an ORM support sub-classing?
Most ORM's reflect fairly close the table-structure of the database they are linked with. As such there is no sub-classing in the sense you mean, other than what follows out of primary and foreign key relationships.
CountZero
"If you have four groups working on a compiler, you'll get a 4-pass compiler." - Conway's Law