# lock Table here
my $obj_rs = $schema->resultset("Table")->search({ col1 => $val });
my $obj = $obj_rs->next;
my $old_val = $obj->col2;
my $new_val = maybe_expensive_computation($old_val);
$obj->col2($new_val);
$obj->update;
# ... fetch and update some more rows
# unlock Table here