I was recently burned by unintentionally breaking backwards compatibility with Perl 5.6 by using use Exporter 'import' rather than adding Exporter to the @ISA array. I realized that I don't have a very systematic approach to coding for backwards compatibility, which got me wondering about what else I should be mindful of and how others handle this problem.

I know about several well-known limitations, of course, e.g.:

• our variable and the warnings pragma not available until 5.6
• open my $fh, ... not available until 5.6
• in memory file handles not available until 5.8

And I know some less well-known limitations (at least, I don't see them mentioned often, so I'm guessing they're less well known), e.g.:

• CLONE not available until 5.8
• 3-argument open not available until 5.6

I wish I had an encyclopedic knowledge of the delta files: perl58delta, perl56delta and so on. The ideal, of course, would be something like RFC: perlfeaturedelta going back for several versions of Perl. (Joost -- did you ever take that further?)

However, in lieu of something that exhaustive being available, I'd love to hear from fellow monks about their real-life experiences and practices.

Some questions for discussion:

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

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

• 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)

• What workarounds or practices do you use to help with backwards compatibility? (e.g. IO::String or IO::Stringy for "in-memory" files)

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