Yeah, that's a huge issue with Perl 6. The language is nowhere near backwards-compatible with Perl 5, and it can't be easily retrofitted to be. So perl 6 is basically becoming its own language in parallel rather than the successor as was originally planned.
It will be interesting to see if PHP goes that route or not.
PHP already kinda did that once, for the PHP6 effort, which was abandoned after many years of being in development. So it's probably not the way to go.
It will be interesting to see if PHP goes that route or not.