It's a tricky situation. Test::Simple and Test::More are very nice to work with, and once you've become familiar with them, and the other test modules that just slot right in, it's hard to go past them.

If I remember correctly (update: no, I remembered incorrectly, as pointed out below), T::S and T::M rely on low-level stuff that was added as of 5.6.0. Backporting them back to even earlier versions would require adding a certain amount of machinery to 5.005, and development efforts on 5.005 are all but dead. Leon Brocard recently released a new patchlevel (5.005_04) in the unique goal of allowing the codebase to continue compile correctly on newer, fussier compilers.

That is, even if you were to endeavour to install T::M on 5.005, I'm not even sure you could succeed.

As long as you create your own tests with the venerable Test module, you'll be fine. That is, test the functionality you want to verify in the module you're upgrading. When you're satisfied with it, send it off to the author as a patch, explaining that it will be useful for other people still running perls from the previous millenium.