I don't see any obvious problems, but I would make your script produce more verbose output so it is clear what is going on (at least for debugging purposes):

open(FILE, "<", "mahi.txt") or die "Unable to open mahi.txt: $!";
while (<FILE>) {
  chomp;
  print "Looking up PART_NUMBER: >$_<\n";
  $sth->execute($_) or die "execute failed: ".$sth->errstr;
  my ($dup) = $sth->fetchrow_array();
  print "  Query returned: >$dup<\n";
  if ($dup eq $_) {
    print "  PART_NUMBER $_ is present\n";
  } else {
    print "  Failed to locate PART_NUMBER >$_<\n";
  }
}
[download]

If you run this script perhaps it will tell you more about what is failing.