The major problems I see are that Perl historically has been a different language with each major revision, although perhaps Perl 6 was too much of a difference.
Could a more conservative approach to Perl 7 be useful, perhaps limiting the breaking changes to making use strict default when parsing from a file (meaning, except for perl -e ... one-liners), making use warnings default in all cases, enabling all non-experimental features from the end of Perl 5 by default, and dropping the Perl 5 experimental features that were not promoted to core?
XS will be "fun" no matter what we do; there are already compatibility shims for XS/perl within the 5.x series. While I think that the burden of introducing "XS2" would be excessively high, it might just be workable if XS2 extensions can still be compiled into XS and used with perl 5 and XS2 gives significant benefits with Perl 7.
The key here is not breaking backwards compatibility without a very good reason and "PBP says that is bad" is not a good reason.
