I am expecting there to be quite a bit of "test fixture setup" that is repeated from one test to the next (mainly setup of mock objects or subroutine overrides), possibly with variations in the setup data between tests.

Ah. Then this is stuff that Test::Class can help with. You can stick all your repeated fixture setup in a (wittily named) :setup method. You can specialise fixtures with subclassing. That said - wait until you get the duplication before you factor it out would be my advice.