What's a regression test?

Currently, how do you test your software? Is it ad-hoc manual testing? Or do you have formal manual test plans you run through? Or do you use automated testing?

As you change your code to use strict, there is a risk that you may accidentally break a working system. If you have an automated regression test suite in place, you can:

1. Run the automated test suite. It passes. Good.
2. Make your code changes to make the code strict-safe.
3. Re-run the automated test suite. If it now fails, you know that your code change broke the system.

Of course, if your automated regression test suite is mickey-mouse, passing all the tests doesn't mean much. But if you have a comprehensive test suite in place it gives you much greater confidence that you haven't accidentally broken the system while cleaning up the code.

BTW, note that Perl itself has a comprehensive automated regression test suite (run with "make test" when you build Perl). This test suite is an invaluable safety net to Perl developers as they change the (fragile) perl C source code.