mod_perl is hard to test because you have to test how your code interacts with the black box that is Apache's guts; how it copes with running lots of copies concurrently; and so on. Yes, it's easier if your code is in nice little modules that are decoupled from mod_perl, but then you're not testing mod_perl. You still need to test to thin layer that sits between your modules and mod_perl and you still need to test the concurrency bit. Programmers aren't used to thinking about concurrency even if they jolly well should be, as can be seen from the vast majority of code on the CPAN which doesn't bother with any concurrent tests at all, let alone really carefully testing it.