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it seems like since Perl 6 is already committed to not worrying about backward compatibility
Perl 6 is promised to be compatible in 80-90% of the code out there. Sometimes it may have a radically different implementation (such as the => operator), but will end up working the same way in practice. Other things have had significant additions, but will still work the old way if you really want to (like subroutine signatures--but realize you'll be missing out on a lot of really cool stuff if you ignore them). Regexen are the only major subsystem I can think of that are complete incompatible.
Note: All code is untested, unless otherwise stated