The tutorial that really helped me with pack and unpack the most was from Perl's POD: perlpacktut. It has a lot of great examples, and a pace that I found interesting but not overwhelming at first.
Update: rob_au mentioned that perlpacktut is only available in Perl 5.8.0 or later. So for those who are still using pre 5.8.0 Perls, you may have to use http://www.perldoc.com to read perlpacktut.