Looking back, I made my decisions for Perl versions mainly on the basis of availability of features and availability in distributions:

• 5.004_xxx - an ugly version of Perl that didn't have some regex quoting features
• 5.005_003 - the absolute minimum I want to work on
• 5.6.x - introduction of lexical filehandles
• 5.8.x - better Unicode support, distributed with Redhat
• 5.14.x - distributed with Redhat
• 5.16.x - fc operator for Unicode fold-case comparisons
• 5.18.x - signatures

I use function signatures in some pet projects and really like them, but so far I haven't found a good enough reason to make modules for general consumption unusable under 5.18, or enough motivation to release my hacked together Filter::signatures, which implements very simplicistic function signatures under any Perl version. I also dread the support questions of people using source filters.