I can think of two good ways to allow Perl 5 programmers to take control of program flow. First, write an alternate runloop that gives language access to the argument stack. It might be necessary to allow alternate dispatch mechanisms (and I'm not quite sure how to handle entering and leaving scopes).
The other option is to replace the body of pp_nextstate and some of the enter and leave ops.
None require core changes. All require XS. Perhaps I need a nap.
