Put use strict; at the top of your program, run it, and see what doesn't work anymore. If it's not obvious from the error message what's wrong with a particular line, look through the secion on strict in the Camel book or the strict perldoc.

You should also get in the habit of turning warnings on, with either the -w flag to Perl or use warnings;. I'd give the same advice for learning how to write warning-free code: turn it on, see what breaks, see why, fix it.