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#!/usr/bin/perl -w
use strict;
...
#...snipped out the original checkUPC here to save space...
print &checkUPC("142687305632"), "\n";
print &checkUPCrobot("142687305632"), "\n";

# returns error message on failure, undefined value on success
sub checkUPCrobot2
...
  my $chk2 = 10 - ($mult%10);
  return "invalid checkdigit...should be $chk2" if $check!=$chk2;
}