Actually, Net::Patricia turns out to be most impressive, giving sub-30 microsecond lookup times against a database of over 203,000 prefixes.

Even using the crude, single tasking, blocking server below, it is able to achieve and sustain a throughput of over 100 queries per second using barely 4% of my 2.66GHz processor, when being queried from 100 clients simultaneously, as fast as they can go. Real impressive. Network latency is likely to be your limiting factor.

#! perl -slw
use strict;
use IO::Socket::INET;
use Net::Patricia;
use Time::HiRes qw[ time ];

my $pat = Net::Patricia->new;

open I, '<', 'IP-to-ASmapping.txt' or die $!;
$pat->add_string( split ' ', $_ ) while <I>;
close I;

my $listener = IO::Socket::INET->new(
    LocalAddr => 'localhost:88888',
    Listen => 1000,
    Reuse => 1,
) or die "Couldn't listen on localhost:88888; $!";

my $time = time();
my $count = 0;
while( my $client = $listener->accept ) {
    printf "\rRate: %7.3f\t", $count / ( time() -$time ) if ++$count %
+ 100;
    chomp( my $ip = <$client> );
    print { $client } $pat->match_string( $ip ) || 'n/a';
    close $client;
}

__END__
C:\test>623661-s
Rate: 108.149
[download]

And the equally crude threaded client firing randomly generated IPs at the server:

#! perl -slw
use strict;
use threads;
use threads::shared;
use IO::Socket::INET;

our $I ||= 1000;
our $T ||= 100;

my $running :shared = 0;

sub thread {
    { lock $running; ++$running }
    for( 1 .. $I ) {
        my $server = new IO::Socket::INET( 'localhost:88888' )
            or warn "connect failed: $!" and sleep 1 and next;
        my $ip = join '.', map{ int rand 256 } 1 .. 4;
        print $server $ip;
        printf "$ip was reported as a member of %s", scalar( <$server>
+ );
        close $server;
    }
    { lock $running; --$running };
}

threads->create( \&thread )->detach for 1 .. $T;

sleep 1 while $running < $T;

while( 1 ) {
    sleep 1 until $running < $T;
    threads->create( \&thread )->detach;
}

__END__
...
86.11.176.198 was reported as a member of n/a
222.75.137.143 was reported as a member of n/a
161.31.232.201 was reported as a member of 21852
93.42.178.58 was reported as a member of n/a
219.220.7.27 was reported as a member of 4538
235.24.7.24 was reported as a member of n/a
227.191.55.46 was reported as a member of n/a
206.129.179.146 was reported as a member of 6347
241.200.47.144 was reported as a member of n/a
114.121.196.146 was reported as a member of n/a
23.138.46.31 was reported as a member of n/a
107.39.33.164 was reported as a member of n/a
183.93.218.229 was reported as a member of n/a
223.233.44.15 was reported as a member of n/a
232.140.217.73 was reported as a member of n/a
203.175.78.140 was reported as a member of n/a
204.118.18.156 was reported as a member of 1239
33.185.129.154 was reported as a member of 721
229.141.204.6 was reported as a member of n/a
66.187.219.196 was reported as a member of 22183
132.103.88.186 was reported as a member of 568
49.161.3.238 was reported as a member of n/a
...
[download]