I would like to put another spin on the 'self taught gaps in knowledge' mentioned above.
in reply to Difference in self v/s instructor based Perl training
Many people who teach themselves a new language (assuming they already know how to program) will simply do exactly what they already know using a new syntax. You don't tend to learn very much that way. Some people will put in the time and really learn something new but that takes extra effort.
A good instructor will force you to do things in a new or different way than you would on your own.