Great explanation, hacker!

I would like to add few considerations to what you said.

• hidden fields make impossible to cache generated pages;
• hidden fields can hold different session_ids for different instances of the same browser, and this can be useful sometimes;
this is impossible using cookies;
• your way to generate session ids can be improved using the code coming right from the Eagle Book:
  

  use MD5 ();
  $MAC = MD5->hexhash($secret .
         MD5->hexhash(join '', $secret, @fields));
  [download]

  
  Now you also have a secret string, not predictable by the client, and double md5, to prevent dirty tricks over digested string. There is a nice introduction about this at page 213 of the Eagle Book.

Ciao, Valerio