Proper Unicode support certainly would be a good reason to not use any earlier version. Unfortunately, perl doesn't have it yet. Unicode needs to "just work" for it to have proper support. It doesn't, as is obvious from a great many posts in this august forum. This isn't entirely perl's fault, of course, as it depends on broken libraries, broken terminals, broken filesystems, broken web browsers (for many use cases), and so on if funny furrin characters are to Just Work. It will be a very long time before Unicode Just Works, because *everything* needs upgrading. So Unicode is a bit of a red herring.
I see only one thing in the various perldeltas which would persuade me that upgrading a working perl 5.6.2 (and re-testing everything, re-installing all my modules etc) to any subsequent version on any of my machines would be worth it. That one thing is userelocateableinc in 5.10, which is useful on one machine. The only other reasons I run higher versions of perl are:
- Specifically for testing purposes;
- Because that's what came with the OS;
- Because it's easier to download the source for $latest from search.cpan.org;
- Because someone whose code I depend on used a new feature when he didn't need to, and I'm too lazy to backport his code