One great text not mentioned is Conway's Perl Best Practices. It just came out from O'Reilly, and has a nice chapter on Object Oriented best practices, including how to make your object encapsulation as secure in Perl as in traditional OO languages (and, in fact, more secure, because Perl is just that cool when you do it right.) Really, a great book for an intermediate programmer, even though nobody will agree with everything in it. You'll learn a few extremely useful things that you won't believe you lived without before reading it.
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