my %IP = (  # create and initialize
    '198.144.186.184' => {
      'domain' => 'host.colocrossing.com',
      'count'  => 1,
      }
    );
...
# add to structure
my $captured_IP     = ...;
my $captured_domain = ...;
$IP{$captured_IP}{domain} = $captured_domain;
$IP{$captured_IP}{count}++;
...
# print structure -- see perldsc