Back up and look at where you're creating that statement handle ($sth, in the $i<$x loop). You're possibly creating several statement handles, you're using them without any error checking, and you're only invoking $sth->finish() on the last one created. Then you're trying to reuse the finished statement handle for the failing do(). I recall Oracle being quite sensitive about finishing statement handles.

So, rewire your logic a bit so that you finish() each handle you acquire, and then acquire a fresh statement handle to do the update.