If you exec a cgi there, you just have to write the CGI. You'll have to get the environment variables and write them to a file. Unless you can guarantee that two people will never simultaneously visit your website, you'll need to use lock files. The environment variables come automatically into the hash %ENV. Using keys, you will be able to identify and process the various variables: (very simplified code without locking)

open FILE, ">>logfile";
foreach keys %ENV {
  print FILE, "$_ => ".$ENV{$_}
}
close FILE;
[download]

Certain environment variables are considered tainted and should be cleared before running this if you wish this to work under the -T flag, as you should.

Of course, your webserver probably records all of this information in a logfile, assuming you have access to the whole server (normally these get stuck somewhere under /var/log on a *nix system, but it could be configured otherwise.)