Aside from the cartesian issue that rdfield and derby note, you might also get a speed increase by using prepare_cached or moving the second prepare outside the foreach loop so that you're not (re)prepareing the same statement over and over (especially silly when you go to the trouble of using placeholders!).

$sth = $dbh->prepare(...);
foreach $po (@po) {
   for ($count=0; $count<=$i;$count++) {
        $sth->execute($old[$count],$po);
        # ...
   }
}
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    --k.