With perl7 still being worked on, is it really a continuation or they're just branching off? I'm still not sure what's their relation to each other now. Why would one choose to use perl over raku or vice-versa?
Perl7 seems to be Perl5 with some changes to the defaults. They can't upgrade to Perl6 as that would be needlessly confusing. If foresight were perfect, Perl6 would have always been a different language name so Perl proper would continue without confusion. The problem was that they had no idea how hard it would be to make Perl6 and thus thought it would be ready in a few years (my understanding) and everyone would upgrade. Raku/Perl6 is SERIOUSLY COOL. I think it just needs a larger community and that's a chicken/egg problem.
Perl 6 was intended from the start to be the next major release of Perl (at various times, a replacement for a 5.10 or 5.12 or 5.14), and it was intended to have a backwards compatibility mode to run 5.8 (or 5.10 or 5.12) code in the same process, with full interoperability.
As time went by, that plan became less and less likely. Some people came up with the idea that Perl and P6 were "sister languages", both to have new major releases. I think this happened sometime around 2009 or so, maybe as early as 2007.
Also by 2011 or so, the P6 developers effectively scuttled the backwards compatibility plan and code written to that point, but I've argued that their plan to replace Parrot was a mistake enough here and elsewhere already. (Sometimes I wonder, now that MoarVM is older than Parrot was when Parrot was declared unsuitable, if they've achieved their promised speed and compatibility goals.)