(The following is mostly supposition, based on my current experience implementing Thread::Sociable)

1. threads::shared is based on tie()s, which don't completely support handles yet (tho 5.10 looks promising)
2. How to deal with multiple threads writing to or esp. closing a handle (more complicated than just refcounting)
3. The fact that the handle needs to be instantiated in the shared interpretter: how does it get there ? perhaps a new open(), e.g., shared_open() ? Or predeclare a lexical handle variable as shared ?
4. Maintaining the illusion of fork() on Windows

I've vague notions about trying to hack something via PerlIO layers and routing thru shared (or rather, sociable) scalars to an I/O object, but need to better understand how the clone operation works. (and based on some recent p5p posts, it appears PerlIO isn't entirely thread-safe).

However, the tie() approach w/ special open() to install things into the shared interpretter is looking more attractive to me (despite the beached whale issue aka the global shared interpretter lock), though its likely to require big gobs of XS/C.