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.
in reply to Re^6: Nobody Expects the Agile Imposition (Part VI): Architecture
in thread Nobody Expects the Agile Imposition (Part VI): Architecture
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.)