how to properly apply lists and hashtables, and that between those two data structures, you can do almost anything. we all learn about data structures in school, but in languages without support in the core syntax, it's often not worth the effort. i came to perl from a mainly C and Java background and in both of those, using a hashtable involves pulling in a library and using an awkward API. the result is that it doesn't feel like a natural way to attack a problem. Perl makes it so easy to use a hashtable that it becomes second nature and you can't imagine going back to a language that doesn't have hashtables built right in.