I'm looking for a generic approach, is_deeply is an example where its not that easy, but reiterating an example with is_deeply would lead the thread again into a different direction.

So either I give a trivial example which is too easy or a complex where people are concentrating on the concrete example instead discussing the abstraction.

> If the latter, Test::Builder::Tester seems designed to test other tests.

Good call! looks promising, thanks. :)

> Are you testing regular code, or testing other tests?

Both. I already experimented with Test::Builder to generate another Test object redirecting the output.