Could it be because you didn't close your sth? Try adding $sth->finish; before $dbh->disconnect;.

Note: The latest DBI docs claim "The finish method is rarely needed, and frequently overused" and is only needed when not all the data is fetched. However, it could be that this isn't properly implemented, since it is a complete reversal of the author's old policy. Older versions of DBI would complained (via a warning) that finish must be called before disconnect, and that allowing finish to be omitted is deprecated and may not be allowed in future versions. (Gee, I wonder why it was "overused".)