Well, one thing I used to forget a lot was to make sure I successfully opened a filehandle properly, cause it'd cause my code to die in a funny way, so I reminded myself to

open FH, "< blah.txt" or die "Can't open file: $!";
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and I could never remember what the special variable was, until I came up with the thought that, when I cause I program to die like this, I'm taking it out with a bang.

<rimshot here>

~Brian