What Discipulus and davido said. I found the Perl Cookbook extremely eye-opening because it’s not theoretical, it’s task oriented. That said, it’s verrrrrrrry out of date so should be taken as a leg up on modern approaches enabled by the CPAN and newer Perls.
The best penny-dropper for me is reading tests. Most distributions have a test directory called t. On metacpan.org you can search and visit a module/package of interest, click on Browse, navigate to the top of the the package ({USER/PACKAGE-VERSION} formatted name currently), click on t, and read tests. Download and decompress to play around, edit, and run yourself with prove.