I'm not sure if you read the readmore section of my posting but I am familiar with the three links you provided and have used both of the test modules that you posted. I do write tests for code deliverables that I am working on.

The piece of development I am referencing is the unforeseen element of code performance in as-yet unguessed circumstances. This often occurs when I combine an old package with some new package to fulfill an additional requirement that was not initially identified during the development of the old package. Another case is when an old package is fed an unforeseen corner case set of data. I know there is a whole science for creating these corner cases for test including Devel::Cover however I am generally lucky to get time to get basic tests in place before other functionality is required. When I find a bug I write a test so the bug won't come back but if I don't know about the bug in advance I move on.