You're right - a simple test case doesn't have any problems. It turns out that our (I'm working with v_thunder on this project) helper function was trampling over $_, which caused the failed assertion only in combination with some other buggy code.

Rewriting the helper to use a different local variable makes the original code work as expected.

I'm having trouble producing a simple case that shows the problem. I think we just happened to hit a case where buggy code happened to work on earlier perls, but doesn't in 5.8. :)