The easiest way to figure out which variable is undefined is probably to print them out with a delimiter:

print STDOUT "|$a|$b|$c|$d|$e|\n";
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If you get

"1|2||4|5"
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You have an answer. If for some reason you can't simply print out the data, try using the perl debugger. You can step through your program line by line, and display the value of any variable at any time.

-Logan
"What do I want? I'm an American. I want more."