I'm confused about what you're trying to implement. A way to learn about hashes? A GUI debugger like the one in Komodo would be better for that. A way to store your notes? You could do that with MySQL.

MySQL is a relational database, and you can implement address books (and other data storage schemes) with it and access its dbs with Perl. Once you know how to use MySQL there are Perl modules mentioned in the other replies you can use to talk to it.

IMO the Perl side is pretty straightforward--you connect to the DB, issue a MySQL command, read the reply into an array or something and use it just like any other data in Perl.

The learning curve is more on the MySQL side, and you should learn that before you try to run it with Perl.