I find these becoming increasingly indisposable -- Path::Class, DateTime, JSON::XS, Template::Alloy, DBIx::Class or Rose::DB; Test::Class. To some degree you can begin to look after authors and see either what they are writing writing or preferring. If Dave Rolsky, Yuval Kogman, or Adam Kennedy--and plenty of others, they just jump to mind--write, use in their code, or recommend something, you should at least check it out. No one bats 1_000 but those cats come close.

The ones you mention are good baselines. The ones I find best practices for modern code are not necessarily wonderful things like like List::Util (note, no "s") but things like DateTime which seem like more/too-much work up front sometimes but save your @$$ when your code explodes or features creep. One of the main advantages of using code like this is it steers your design. When you have well designed OO code you can't jury-rig/wing-it as much. You are forced to be more disciplined by the tools.

Update: Perl 5 Wiki: Recommended CPAN Modules.

Update 2: I'm going to also mention URI and say that this site and the external journals of some of its members are probably the best thing you can do to get back into Perl in a way that will make you happy. Many of the users here give gospel as much as advice. I really believe a year at PM paying attention and participating is worth more than some degrees and certificates.