I'm a little late to the party, but I don't think the
problem of compatability will be as bad as you think. In
the first Apocalypse Larry says

"..the default in the main program (and in one liners) is Perl 5..."

It isn't clear yet what will clearly declare a program as Perl 6. He suggests maybe a 'use 6.0;' pragma or a more general 'use Policy;' pragma, but he left it up in the air. The section of Apocalypse one on "Keep Perl free of constraints" has some more on this and Larry seems completely aware of the fact that it's a pain in the ass to port scripts. Not to mention all the one liners that have made thier way into shell scripts.

Tye's comment on garbage collection sounds like much more of a concern to me.

Ira,

"So... What do all these little arrows mean?"
~unknown