The CGI thing has bitten me often enough that I've learned to avoid it, but I still have the teeth marks.

What I also tend to get wrong sometimes is the difference in precedence between or and ||, especially in return statements. Usually when I pretend to be terse and write:

sub is_foo_or_bar { return foo($x) or bar($x) } # no! use || here
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I don't do this often, but when I do, it can take a long time to debug, because that code looks deceptively sensible.