I don't think backwards compatibility necessarily means a bloated and unweildly system. And I don't think that's the reason why Windows is so big. Linux maintains compatibility for old hardware (in some cases, must older than Windows), and yet it doesn't suffer from the same issues.

If you're talking about newer versions of Perl retaining hardware compatibility (I don't think you are) then this is a good analogy. Perl is like Linux in that it can be compiled to suit your achitecture.

If you're talking about Perl having backwards compatibility at a language level, then I don't think this is a huge issue. There's no reason why we can't have Perl5 and Perl6 working side by side. In fact, I believe this is exactly what Parrot is for.

Perl6 will gramatically be a completely different language, however I believe (and I don't know a lot about P6, so hopefully someone will correct me if I'm wrong) that most of CPAN will work with it (one of the main reasons for the Phalanx project).