Just thought I'd throw in something else that I haven't seen posted yet: saying

use strict;
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lets anyone else who reads your code know that you know what you're doing, which makes a difference when they read it. (They can, for instance, assume that if they see an unqualified variable in a subroutine, it's local to that sub, and not some sort of magic state that affects other parts of the code.) This makes your code easier to read, which makes everyone happier.