I've edited my original post to make the code downloadable and written a step by step test case.

I've eliminate all but the defaults from Catalyst and KiokuDB by making the test case create a new application from scratch. The test case doesn't prune Catalyst further as it should (in theory) work just like that.

I've noticed the comment in Catalyst::Model::KiokuDB::setup_scope_guard() too. But Devel::Monitor claims that all variables declared in setup_scope_guard() and the callback in Scope::Guard are freed. Obviously, something isn't freed probably, but I'm starting to think it isn't an object, I'm looking for.

Devel::Monitor does something very similar to overriding DESTROY, only it does it by using some tie-magic, I think. But the result is the same; tracking when an object gets destroyed on-screen. Anyway, I'll try overriding DESTROY for a few candidates, just in case.