Beware that $sth->rows() will not work in this case for many (most?) DBI drivers. From "perldoc DBI":

Generally, you can only rely on a row count after a non-`SELECT' `execute' (for some specific operations like `UPDATE' and `DELETE'), or after fetching all the rows of a `SELECT' statement.

For `SELECT' statements, it is generally not possible to know how many rows will be returned except by fetching them all. Some drivers will return the number of rows the application has fetched so far, but others may return -1 until all rows have been fetched. So use of the `rows' method or `$DBI::rows' with `SELECT' statements is not recommended.