Thank you for your quick and thorough reply. After looking through your revision I can quickly see how my nested control structures were exaggerated nearly to the point of obnoxious. In particular I really liked your use of:

    for my $i (map {$_ * 2} 0 .. $#cardNum / 2) {
        $cardNum[$i] *= 2;
        $cardNum[$i] -= 9 if $cardNum[$i] >= 10;
    }
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to cycle through @cardNum. I had thought of trying to use map, but wasn't quite sure how to pick every other element. The only part I don't quite understand is:

   return <<INFO;
    
   Card Number: $cardNum
   Issuer:      $issuers{$iDigit}

INFO
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I can see that the subroutine is returning a block called INFO, but I don't quite get the syntax behind the way you defined that block. Is there a name for this style of definition that I can research for a better understanding? Thank you again for reviewing my code. I'm a bit disappointed with myself looking back, but I suppose sophisticated code comes with time and practice.

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