EvanK:

Mine still splits & loops. I made only small changes:

#!/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";
[download]

I first got rid of the if even/odd in the first while loop by shifting the values off the array in pairs. I got rid of the second while loop by direct calculation.

UPDATE: I fiddled with it a little longer and came up with a slightly improved model:

# returns error message on failure, undefined value on success
sub checkUPCrobot2
{
  # 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;
}
[download]

--roboticus