my $last;
my $count;
while (<>) {
    my $ip = extract_ip($_);

    if (defined($last) && $last ne $ip) {
        print("$last: $count\n");
        $count = 0;
    }

    $last = $ip;
    ++$count;
}

if (defined($last)) {
    print("$last: $count\n");
}