Well, in the code you presented, I see that '$sth2->rows()' is called, and later on, '$sth->finish()' is called. So you have two statement handles open at the same time.

The SQLite Home Page has a page that describes common SQL features not implemented in SQLite. The list is short, but one item on it says, "Nested Transactions: The current implementation only allows a single active transaction." See http://www.sqlite.org/omitted.html.