• use warnings. Being able to do proper lexical scoping with your warnings is very nice.
• Large files. Very important if you ever need to work with a file larger than 2 GB (you will need this at some point)
• 64-bit support. You can get this either if your processor supports it or if your C compiler can fake it (of course, it's not as fast on 32-bit archs)
• POSIX character classes. I don't use these often, but when I do, I'm glad I have them, as the simple character classes Perl has always provided are sometimes not quite enough
• our. There is a little controversy around this vs. use vars, and in general it's better to use my over either of those, but there it is
• qr. Kill off messy /o regex modifiers forever

Most of these came in 5.6.x. If you're going to upgrade from 5.005, you might as well go all the way to 5.8.