If requirements don't dictate a version of Perl to target, what version do you target when writing code for reuse?

I usually target the last three stable releases, unless requirements point me in another direction. So currently the earliest I worry about for my modules is 5.8.5.

Put differently, what features are you willing to give up for backwards compatible code?

Unless the requirements force me to, I'm usually not willing to give up features. In most cases I've found it more effective to spend the effort to get the requirements changed rather than stop using features I find useful.

What features/code/modules that break compatibility do you most frequently encounter or have burned you the most, particularly when other, compatible methods would do just as well? (In your own coding or on CPAN)

I guess the one that causes me the most pain, since I can't use Test::Class without them, is the lack of attributes in earlier Perl's.

What workarounds or practices do you use to help with backwards compatibility?

Unless it's forced by the requirements - I don't :-)