Is this required by the language
There's nothing in the language that precludes a 5.0005-style threading implementation. The difficulty was in retrospectively trying to make the existing implementation thread-safe, where it had never been designed for that possibility. This is one of the (many) reasons why it was concluded that a complete from-the-ground-up rewrite of the perl interpreter was required, i.e. perl6.
The main drawbacks of the ithreads model are: that cloning the existing interpreter when creating a new thread is slow;
that it uses lots of memory, since the new interpreter doesn't make any use of the OS facilities that a fork() would, of sharing memory by default with copy-on-write pages; and that having shared variables is slow, clunky and is memory-heavy.