For tracing progress, watching variables, and adding development time assert etc. in code in a way that you can disable easily (change one line), for production and know that they will have zero effect on it's performance, take a look at Devel::StealthDebug.

Cutsey name (though I cannot think of a better one), and it is a source filter, but the source filtering only happenss when enabled, so not in your production code. The statements remain behind as simple comments ready for when the bug reports arrive.

I'm not sure about how it would interact Test::Harness.