It's already been mentioned, but Randal L. Schwartz's Learning Perl, Chap 5, is a must. So, are the first 9 chapters of Programming Perl (hashes are covered in Chap 9, but then the sledding gets heavy). But I never fully understood hashes until I read the aforementioned Randal L. Schwartz's Perl Objects, References & Modules. The book isn't really about hashes, but he covers a lot of that foundational stuff in a very illustrative and easy to understand way. Pretty soon you'll be talking about HoA's, AoH's, andHoH's--but remember, they're all just scalars.

Welcome to the monastery. And by the way, Matins are sung at 4:00am so get to bed early.

Update:
How could I forget the Perl Cookbook. It's Chap 5 also has a short introduction to basic hashing, but then some simple to complex practical examples. We are so fortunate in Perldom for so many get resources.