This is a very old question, discussed at some length by Martin Fowler in his book "Patterns of Enterprise Architecture." For a brief practical discussion, take a look at the Tangram docs for mapping inheritance. Tangram is the only Perl O/R mapper to handle this, AFAIK.
That link to Tanagram::Relational::Mappings was very useful indeed - providing a list of the pros and cons of the models I described above. Thanks for that.
The Martin Fowler book Patterns of Enterprise Application Architecture looks interesting - think I'll give it a read. As somebody with almost zero Java experience, would it be impenetrable or reasonably easy to map to Perl?
The text of the book is pretty good, but the actual code examples will be hard to follow if you've never used Java. You can take a look at some sample writing from the book on Fowler's website.