Over on Backcompat is holding us back!
"Breaking XS would be awful, but it's probably necessary for the kinds of internal changes that produce either dramatic speed gains or allow further experiments on the CPAN. Unfortunately, it breaks most of the CPAN."
And Stevan Little replies:
"Actually the XS problem is easily fixed (for some definition of easy). Stop exposing the Perl guts via a macro layer and provide a real API (aka - level of indirection). Of course, then you need to convert all XS modules over to use it, but with a sensible deprecation cycle (of like 2+ years) that should be do-able (not to mention that it will shake out all the old crusty XS modules that no one maintains or cares about)."
It could be that Moe and Perl6 are hitting similar issues with Perl5 integration/compatibility and they may have a common resolution.