The point isn't so much to unlearn anything as it is to learn new ways of doing the same old things. One example that has been given is using map() instead of blithely iterating over an array. I personally had to get used to using postfix conditionals (XYZ if(foo);) for one-statement conditional statements instead of just sticking the statement to be executed on the next line.
As a matter of fact, PERL helped me understand some of the finer points (no pun intended, it'll make sense later) of other languages. It wasn't until I grokked PERL's references that I really understood the power of C's pointers (makes sense now, doesn't it? *ducks flying objects*).

~Cybercosis

nemo accipere quod non merere