my $sth = $dbh->prepare($sql_last);
# ...
while (my $pointer = $sth->fetchrow_hashref){
    my $sql_back = '...';
    my $sth = $dbh->prepare($sql_back);
    while (my $pointer = $sth->fetchrow_hashref){
        # ...
        my $sth = $dbh_2->prepare($sql_net);
# ...