Turning that into an iterator sounds like an interesting task, but it's hard to do without having code that actually runs. So your best bet for getting a complete solution is to provide a complete script that constructs such a tree structure, runs the sub against it, and compares the output to the desired result, preferably using Test::More or something along these lines.

I will be offline for a few days, if nobody else answers in the meantime I'll do it ;-)

If you just want to return some results from a sub and then continue running that sub, you can take a look at Coro, which allows you to do just that (or, to advertise my own modules, Perl6::GatherTake, which is just a nice wrapper around some Coro functions).