The bug (and other reported curruptions) are definitely coming from the
COW logic in register.c. This is what happens:

Setting up the exception handler (which is a continuation) triggers COW
setting of the register stacks. Then on first subroutine return from
__inner, the register stack is unCOWed (the chunk is copied). When then
returning from __outer stack->top->used is still 2 end the same chunk
gets popped off the stack, because the interpreter's stack is still
unchanged.

The register restoring memcpy is the same *twice*:

memcpy (dstpp=0x824ca50, srcpp=0x4017d9a0, len=64)

So returning from __outer places P17 (the RetContinuation of __inner) in
mains registers and ruins the PerlUndef that you wanted to print.

Just returning from mark_register_stack_cow() makes the bug vanish

(but doesn't fix anything)

As mentioned several times, a COWed buffer needs distinct buffer headers
and shared buffer memory. The current implementation in register *and*
other stacks is broken. *Fixes welcome*.

You can currently avoid the bug by not using Continuations and Exception
Handlers.

Thanks for your test program,
leo