There's another interesting way to do this called mock objects. A brief description: "We propose a technique called Mock Objects in which we replace domain code with dummy implementations that both emulate real functionality and enforce assertions about the behaviour of our code." (From Endo-Testing) It's Java-focused, but since Perl has a great tradition of borrowing interesting tools and ideas from other languages I don't think that is a problem ;-)
Since I tend to approach problems like this from a code generation standpoint, I find this very intriguing. What if we were to create metadata to configure how the different methods in our objects are supposed to work? Then the code that uses these objects can always depend on a consistent environment and we can test the interesting stuff -- processes that use the objects -- with full confidence.
I haven't implemented anything with this idea yet, but it -- along with generating most of my unit testing code -- has been in the back of my mind for a while now. Testing sizable data-backed systems is hard, and any boost will be greatly welcomed.