(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)

better:

        if (inet_pton(AF_INET, $ip)) {
                # AF_INET
        } elsif (inet_pton(AF_INET6, $ip)) {
                # AF_INET6
        } else {
                # not an IP address
        }