I think you are looking at Class::DBI from the wrong angle - Class::DBI is not a tool to force existing tables (and their relations) into classes/objects, but to provide some simplicistic way of storing objects in tables.
My thoughts were the exact opposite -- Class::DBI is an abstraction layer that let's you interact with your tables as objects. Throughout the documentation, when the author refers to 'objects' he generally means 'rows'.
Not that you can't do that. I'm only saying that Class::DBI is not going to necessarily make it easer to represent class inheritance in a relational system.