sub checkMAC {
    my $mac = shift;
    my $httpThr = async {
        return head "http://$mac:80"; ## Simplified!!
    };
    my $authResult = get 'http://AuthServer/Check.pl?$mac'; ## Simplified!!
    my $httpReult = httpThr->join; ## join immediately
    lock %macs;
    $macs{ $mac } = $httpResult && $authResult ? 1 : 0;
}

##</code><code>##

while( <LOG> ) {
    next unless m[ ACK \s+ ( (?: [0-9a-f]{2} : ){5} : [0-9a-f]{2} )]xi;
    async( \&checkMAB, $1 )->detach; ## detach cos there's no result we're interested in
}