$sth=$dbh->prepare("
    UPDATE tableA
    SET colA =( SELECT colB FROM tableB WHERE colC=? )
    WHERE tableA.rec_no=?
");
foreach my $row ( @rows ) {
    $sth->execute(
        $row->{'tableB.value'}, $row->{'tableA.value'}
    )  or die $dbh->errstr;
}