my $sth = $dbh->prepare('update data_set2 
                         set data = ? where key2 = ?');
$sth->execute($val1, $key);

##</code><code>##

SELECT id_field, field_to_update 
    FROM table_1 WHERE id_field NOT IN
    (SELECT join_id_field FROM table_2)

##</code><code>##

SELECT src.id_field, src.field_to_update
  FROM table_1 src, table_2 dest 
  WHERE src.id_field = dest.join_id_field 
  AND src.field_to_update != dest.field_to_be_updated