perl-Return-MultiLevel (return from nested callstack)

This module provides a way to return immediately from a deeply nested
call stack. This is similar to exceptions, but exceptions don't stop
automatically at a target frame (and they can be caught by intermediate
stack frames using eval. In other words, this is more like
setjmp(3)/longjmp(3) than die.

Another way to think about it is that the "multi-level return" coderef
represents a single-use/upward-only continuation.
