Are your "very good reasons" good enough to justify using a threading implementation which is experimental and is known not to be thread-safe?
Of course ;p I am aware of the threading stability issues, including those affecting non-threaded functionality in the threaded perl interpreter. (Eventually, probably long before perl6 :) that'll become less of an issue.)

Hmmmm, as to your suggestion.... not robust but a good idea. I wonder if i can pass 2 tie()'d filehandles to the client code (replacing STDIN & STDOUT), with explicit functionality to stop the close() from working, replacing it with some cond_wait() on a per-thread scalar.... hmmmm, an async() block cotaining a timer either will or wont have locked that scalar.

Ewwww, it sounds just evil enough to work!
Thanks I'll try :)