my $order_by_size_rs = $all_subnets_rs->search({'source'=>$source},{'order_by'=>{'-asc','mask'}});

while( my $network = $order_by_size_rs->next )
{
    my $overlaping_nets_rs = $net_rs->search({
        'mask'          =>  { '>',    $network->mask()              },
        'bitpattern'    =>  { 'like', $network->bitpattern().'%'    },
    },
    {
        'order_by'=>{'-asc',['mask','start']}
    });

    if( $overlaping_nets_rs->count )
    {
        printf "Subnet %s/%d (%s) has %d overlaps:\n", 
                $network->ip, $network->mask,
                $network->name, 
                $overlaping_nets_rs->count;

        ... # Code to add the overlaps to the report.
    }
}