Reading up on it now, Perl 6 is a specification and Rakudo is an implementation. If you want to write Perl 6-compliant code, you can use Rakudo. "There are others...", but it seems silly to me...

As an outsider, my 2c is "abandon 'perl 6', too much baggage for reuptake, and go with Rakudo, which is a more modern name". People smarter than me decided on this rakudo/perl 6 nomenclature split, but it just feels like an own-goal to me.

https://en.wikipedia.org/wiki/Rakudo_Perl_6