Apart from offering a couple of references:

• Perl 6 S11 (see Versioning section)
• Discussion of Perl 6 versioning on perl6.language
• Python's back to the __future__

I don't feel qualified to comment on this difficult topic ... though I'm keenly interested in it. I'm sure @Larry drew lessons from the CPAN, Python's __future__ module, and other versioning approaches in designing the new Perl 6 versioning features. I'd be interested to learn which versioning prior art most influenced their design.