At the risk of sounding redundant, wouldn't perl -w catch the use of $avg when undefined, thereby making the debugging easier? All the debug tips are great, but the best one is letting perl debug for you with use strict; and -w.

On a stylistic note, you might want to scope your variables like this:

my ($a, $b, $c, $d);
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Instead of this:

my $a;
my $b;
my $c;
my $d;
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This should help keep your code from getting diluted with relatively less important text. (The computation is more important than the declarations, so it should also take up more space on screen to help focus your mind.)

-Ted