my ($sth, $sum) = ($dbh->prepare ("select sum (whatever) from ..."));
$sth->bind_columns (\$sum);

while ( long running loop ) {
    if ($sth->execute and $sth->fetch) {
        # My $sum is now valid
        }
    else {
        print STDERR "Something went wrong: ", $sth->errstr, "\n";
        }
    }