Perl 5. Lots of good code out there to learn from, which will take a while for Perl 6 to be written. And then you have a well documented mental migration path

Perl 5. Lots of good code out there to learn from, which will take a while for Perl 6 to be written. And then you have a well documented mental migration path to Perl 6 since it tends to be explained in terms of deltas to Perl 5.