I'm extremely impressed you thought of checking for private IP addresses - everyone seems to forget that. But as you've described it, there's a time-of-check/ti

I'm extremely impressed you thought of checking for private IP addresses - everyone seems to forget that. But as you've described it, there's a time-of-check/time-of-use vulnerability: an attacker could set a really low TTL on the A record and swap it out with a private address between your check and the time you actually hit the URL. You really have to hook into the HTTP client for that check (in Perl, I recommend LWPx::ParanoidAgent; if using libcurl, you can use a CURLOPT_OPENSOCKETFUNCTION callback).