Perl: Fix DBI::db->disconnect invalidates 1 active statement handle

Add this subroutine to the module:

sub DESTROY {
    my ($self) = @_;
    # Finish off our statement handle, so we don't get warnings like:
    # DBI::db->disconnect invalidates 1 active statement handle
    my $sth = $self->{data_handle};
    if (blessed($sth) && $sth->can('finish')) {
        $sth->finish;
    }
}