Well, if you studied Learning Perl you should know about hashes already (it has a whole chapter on them).

Without a doubt, the next thing on your list should be references.

After that, the next thing I would recommend is to get a good handle on the whole Perl module/package/importation/object/class Ball O' Wax, with a bit of symbol tables thrown in. By way of exercise, collect the simplest ways to create a module, a class, a package, an object (e.g. my $o = bless +{}, 'x'; there, I just created a Perl object belonging to class x, and stuffed it in $o). This will help you sort out these various concepts, which are often presented all at once in a big gemisch.