Perl 5 was clearly a rewrite of Perl 4. Sure, a lot of scripts "just worked" with both. But the interpreter was almost completely rewritten and the official list of known things that could go wrong was rather long.

Perl 6 is more complicated. If there was any hint of compatibility with Perl 5, then I'd call it a rewrite. But they are different languages. And going forward I expect Perl 5 and Rakudo to co-exist indefinitely. (Though eventually I hope that Rakudo reduces confusion by stopping calling itself Perl 6.)