If you're talking about getting the IP address of the browser for the particular connection you are dealing with it is just:

use Apache2::Connection; 
use Apcache2::RequestRec; 

my $client_ip = $r->connection->remote_addr;
[download]

If you're wanting all of the IPs currently connected to all of your Apache children, then you are going to need to write a special handler that will log ( to a file or database ) when a connection comes in, and then remove it again during a Cleanup phase.