I personally don't care how other people write it—there's something to be said for people writing it every way to help Google searches find
—but I always write it "Perl 6" because I think it looks better, and because in ten years I want the name of the language to be "Perl", not "Perl6". The "6" is officially just a version number, because the party line is that it's the same language no matter how much pretzelization we perform.