One way to look at things is that Perl 6 breaks backwards compatibility with Perl 5. Another is that Perl 6 has well designed language interop and evolution features that make it so easy to nicely use code and libs from other languages that backwards compatibility becomes irrelevant.
The big problem with Perl 6 has been that the implementation has trailed the ambitions of the design. But that's changing due to pieces like Inline::Perl5 falling in to place. (See my comments nearby for more details.)
Well, not quite irrelevant. Just because language interop is easy doesn't make the fact that there doesn't exist the interop for a library you need. That said, it's also a good chance to get good sane default implementations in first. On of the problems with CPAN is finding the best of the multiple modules available that fit the need.
As I said elsewhere in this thread, Inline::Perl5 hasn't been tested against all 130,000 modules on CPAN to my knowledge, but, based on what I know of the tech and its 100% success rate so far, I'm going to assume until I hear otherwise that it already works, or will do so this year, for the vast majority.
In the unlikely event there's a module that doesn't work, then yes, you have to learn and wield a new language that isn't backwards compatible. But it is Perlish, and it is designed to coexist beautifully with Perl 5.
Ah, I had heard that Inline::Perl5 worked with (some?) XS modules, but wasn't sure I was remembering correctly, and assumed it didn't. Unfortunately, my ability to keep abreast of new Perl 6 developments has taken a big hit the last six months or so, due to increased workload. :(
The big problem with Perl 6 has been that the implementation has trailed the ambitions of the design. But that's changing due to pieces like Inline::Perl5 falling in to place. (See my comments nearby for more details.)