Seems like this is more about test planning than Perl, but that's OK by me...

demerphq has the right idea about groveling through the Perl delta and your own code, but there might be an easier way.

This'd be easier with a little more knowledge of the nature of the system under test, but I'll take a stab at two test methods you might consider. I'll assume (since it's Perl on Windows) that your production environment is small enough to be emulated in some way in a test environment.

The first approach: emulate your production environment in a test environment running 5.8. Duplicate the input to the production environment (or a subset of that input) and check for whatever's important: speed, data integrity, whatever.

The second approach: set up separate 5.6 and 5.8 test environments and send the same input through both environments. Compare the important stuff in each environment.

This kind of test planning is more art than science, but it may simply be enough to compare representative performance rather than plan particular tests. In any event, I for one am very interested in what you decide to do, and I hope to read more about your testing efforts.

Hope that helps...