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Test::Builder 1.5 is going to break a lot of older versions of existing modules which users might have installed.
Keep a list of modules which had to be upgraded (only those with runtime problems, not problems in their tests). At install time, check them and report any which will break and should be updated. If any exist, issue a prompt asking if the user wishes to proceed (default no).
Keep a list of known broken modules and their first working 1.5 version here.
Test::BDD::Cucumber (broken in 0.12)
These modules will also break, new versions for them haven't hit CPAN yet (and possibly haven't been completely fixed yet).
Test::WWW::Simple also breaks. I don't know which version will be the first to support it yet because I haven't even submitted the patch to the module yet.