#!/usr/bin/perl -w
use strict;
use warnings;

# returns error message on failure, undefined value on success
sub checkUPCrobot
{
  # grab and immediately split upc into array, 1 char per element
  my @chars = split(//, shift);

  # return error message if incorrect length
  if( $#chars != 11 )
  { return "should be 12 digits in length"; }

  # loop through to seperately sum even and odd chars
  my $odd=shift @chars;
  my $even=0;
  foreach (0..4)
  {
    $even += shift @chars;
    $odd += shift @chars;
  }

  # calculate correct check digit
  my $mult = ($odd * 3) + $even;
  my $check = 10 - ($mult%10);

  # return error message if wrong check digit was initially given
  if($check != shift @chars)
  { return "invalid checkdigit...should be $check"; }

  # otherwise, if validated, return undefined
  return;
}
#...snipped out the original checkUPC here to save space...
print &checkUPC("142687305632"), "\n";
print &checkUPCrobot("142687305632"), "\n";