I see a long period where Perl 5.6 and 5.8 scripts co-exist with Perl 6 scripts.

Hey, I'm still waiting for Perl4 to go away.

Kidding aside, I think we'll see good reasons to write brand new code in Perl5 long after Perl6 is out. I suspect Perl6 won't scale down to simple tasks as well as Perl5 does (though it will almost certainly be better than C++ or Java). Most of the improvements in Perl6 are to get it to scale up better.