Thanks for your informative reply.

If I had know that mod::Net::IPTrie or mod::Tree::Trie existed I would have use them, unfortunately I did not know they existed and did not know the terminology to search for them on CPAN, and I thought it would take to long to implement something myself.

Instead I wrote some code using string representations of the binary bits in a database using DBIx::Class.

My DBIC table defintion looks like this:

__PACKAGE__->table('tblSubNet');
__PACKAGE__->add_columns(
    'id'          => { data_type=>'int',          is_auto_increment=>1
+ },
    'ip'          => { data_type=>'varchar', size=>15                 
+ },
    'mask'        => { data_type=>'int'                               
+ },
    'name'        => { data_type=>'varchar', size=>255, is_nullable=>1
+ },
    'start'       => { data_type=>'int'                               
+ }, # the IP address as A 32 bit number
    'bitpattern'  => { data_type=>'varchar', size=>32                 
+ },
);
__PACKAGE__->set_primary_key('id');
__PACKAGE__->add_unique_constraint(['bitpattern']);
[download]

Once I have populated the table of subnets, I then search it like this:

my $order_by_size_rs = $all_subnets_rs->search({'source'=>$source},{'o
+rder_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.
    }
}
[download]

Using this algorithm I was able to search through the 50_000 subnets searching for overlaps in about 10 minutes. (On a 3GHz Linux box).