sub checkUPCGrandFather2 {
    my $str = shift;
   
    return "should be 12 digits in length" if length ($str) != 12;
   
    my $t = 0;
    my $sum = 0;
   
    # loop through to sum even and adjusted odd chars
    for (0..10) {
        my $chr = substr $str, $_, 1;
        $sum += ($t ^= 1) ? $chr * 3 : $chr; 
    }
   
    # calculate correct check digit
    my $check = (10 - $sum % 10) % 10;
   
    # return error message if wrong check digit was initially given
    return ($check != substr $str, 11) ? "invalid checkdigit...should be $check" : 'Ok';
}

##</code><code>##

checkUPCGrandFather1: invalid checkdigit...should be 6
checkUPCGrandFather2: invalid checkdigit...should be 6
checkUPCroboticus: invalid checkdigit...should be 6
checkUPCEvanK: invalid checkdigit...should be 6
                Rate GrandFather1        EvanK    roboticus GrandFather2
GrandFather1 21630/s           --         -11%         -19%         -72%
EvanK        24183/s          12%           --          -9%         -69%
roboticus    26595/s          23%          10%           --         -65%
GrandFather2 76965/s         256%         218%         189%           --

##</code><code>##

use warnings;
use strict;
use Benchmark qw(cmpthese);


print 'checkUPCGrandFather1: ' . checkUPCGrandFather1('064200115897') . "\n";
print 'checkUPCGrandFather2: ' . checkUPCGrandFather2('064200115897') . "\n";
print 'checkUPCroboticus: ' . checkUPCroboticus('064200115897') . "\n";
print 'checkUPCEvanK: ' . checkUPCEvanK('064200115897') . "\n";

cmpthese (-1,
    {
        GrandFather1 => sub {checkUPCGrandFather1 ('064200115897')},
        GrandFather2 => sub {checkUPCGrandFather2 ('064200115897')},
        roboticus => sub {checkUPCroboticus ('064200115897')},
        EvanK => sub {checkUPCEvanK ('064200115897')},
    }
    );

sub checkUPCEvanK
{
  # 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"; }
 
  my ($odd, $even);
  # loop through to seperately sum even and odd chars
  foreach (0..10)
  {
    if($_ % 2 == 0)
    { $odd += $chars[$_]; }
    else
    { $even += $chars[$_]; }
  }
 
  # calculate correct check digit
  my $mult = my $check = ($odd * 3) + $even;
  while($mult % 10 != 0)
  { $mult++; }
  $check = $mult-$check;
 
  # return error message if wrong check digit was initially given
  if($check != $chars[11])
  { return "invalid checkdigit...should be $check"; }
 
  # otherwise, if validated, return undefined
  return;
}

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

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

  my ($even, $odd, $check);
  foreach (0..5) {
    $odd += shift @chars;
    $check = shift @chars;
    $even += $check;
  }
  my $mult = 3*$odd+$even-$check;
  my $chk2 = 10 - ($mult%10);
  return "invalid checkdigit...should be $chk2" if $check!=$chk2;
}


sub checkUPCGrandFather1 {
    # grab and immediately split upc into array, 1 char per element
    my @chars = split //, shift;
   
    return "should be 12 digits in length" if @chars != 12;
   
    local $| = 0;
    my $sum = 0;
   
    # loop through to sum even and adjusted odd chars
    $sum += $chars[$_] * (--$| ? 3 : 1) foreach 0..10;
   
    # calculate correct check digit
    my $check = (10 - $sum % 10) % 10;
   
    # return error message if wrong check digit was initially given
    return $check != $chars[11] ? "invalid checkdigit...should be $check" : undef;
}


sub checkUPCGrandFather2 {
    my $str = shift;
   
    return "should be 12 digits in length" if length ($str) != 12;
   
    my $t = 0;
    my $sum = 0;
   
    # loop through to sum even and adjusted odd chars
    for (0..10) {
        my $chr = substr $str, $_, 1;
        $sum += ($t ^= 1) ? $chr * 3 : $chr; 
    }
   
    # calculate correct check digit
    my $check = (10 - $sum % 10) % 10;
   
    # return error message if wrong check digit was initially given
    return ($check != substr $str, 11) ? "invalid checkdigit...should be $check" : 'Ok';
}