Well I hate to get all academic-y but I'm a big fan of using complexity metrics for measuring subroutines/methods/functions. The higher the complexity metric (ie, more conditionals) - the harder it is to test and that's a good reason to refactor a subroutine. As for modules ... well just like the maxim says, take care of your subroutines and your modules will take care of themselves.

Update: CPAN continues to amaze me.