Your confusion lies in thinking, as you've said, "just a new version of a language." It's actually been a long, 15-year journey that resulted in a completely brand new language. The name is still 'Perl 6' for historical reasons, but it may as well be called 'Rakudo'; something people've not heard of yet. The Perl 5 language is still being developed and improved, with a major release every year. Hence, the filename distinction.

However, it's just a convention. You can have any extension you like (at least on *nix).

And to answer your question, no "use v6" is not mandatory. It's there so if you run your script with a perl (and not perl6) interpreter, you won't get confusing errors.