As long as you don't mean blindingly fast and huge. You want Net::Netmask.

#!/usr/bin/env perl
use strict;
use warnings;

use Net::Netmask;

# our owners and their ranges
my @blocks = (
    [ 'bob', '192.168.1.0/24' ],
    [ 'nancy', '192.168.1.12' ],
    [ 'ralph', '192.168.2.0/255.255.255.0' ],
    [ 'bob', '192.168.3.0/24' ],
    [ 'NOBODY', '0.0.0.0/0' ],
);

# populate the table with tagged blocks
for my $block (@blocks) {
    my ($owner, $range) = @$block;
    my $nb = Net::Netmask->new2($range);
    $nb->tag('owner', $owner);
    $nb->storeNetblock();
};

# do the lookup.
while (my $ip = <DATA>) {
    chomp $ip;
    my $block = findNetblock($ip);
    print "$ip belongs to ", $block->tag('owner'), "\n";
}

__DATA__
192.168.1.45
192.168.1.12
192.168.2.4
192.168.3.33
192.168.4.12
[download]

Output:

$ ./net_netmask.pl 
192.168.1.45 belongs to bob
192.168.1.12 belongs to nancy
192.168.2.4 belongs to ralph
192.168.3.33 belongs to bob
192.168.4.12 belongs to NOBODY
[download]

This works rather well unless you're trying to grok the full internet route tables or such. (Somewhere there's a XS module that implements the routing table from BSD if you need really fast/big).