Simply said, the difference between 5.8 and 5.10 is small. There are some nifty new features but if you don't write "use 5.10" in your code you can't even use them.

That's not true. Some of the new features, even some nifty ones, are always available in 5.10. '//' for instance, and the new regexp features. New keywords however, which could conflict with existing code are only enabled if you use '-E', 'use 5.010' or 'use feature qw(...)'. This include 'state', 'given/when' and 'say'.

But that doesn't mean your perl5 code won't run anymore when perl6 is here, because perl5 will still be available as one of the languages (together with perl6) that all compile down to a virtual machine called parrot.

That's only true if you run Perl6 on an implementation that uses Parrot. Pugs for instance runs on top of Haskell and doesn't need Parrot (But I don't keep up with Pugs, it may very well be that it now has the ability to emit one of the Parrot internal languages). Of course, someone has to to make perl5 run on top of Parrot as well - currently it doesn't, and the only project I know of that tried to do this (Pony) died a slow death several years ago. Perl1 has been ported to Parrot though. Oh, and I haven't heard any solution of how to port XS code to Parrot short of rewriting it.