How would the Perl 5 => Perl 6 transition compare to the Python 2 => Python 3 transition?
The idea for Perl 6 was the same: Clean up the language, add new features at the expense of compatibility, and create a v5 → v6 transition tool. During the evolution of Perl 6, this original goal became less important. Nowadays, Perl 5 and Perl 6 are seen as two sister languages in the same family, which take inspiration from each other, but are independent. There won't be mass migrations to v6 once it “arrives” (it already did in the form of Rakudo Star). There are, however, efforts to run v5 and v6 code in the same VM.