In a cgi environment such as that created by Apache's mod_cgi, the only real difference between a POST request and a PUT request is the contents of $ENV{REQUEST_METHOD}. Do a PUT and a POST request to /cgi-bin/printenv.pl (in a stock Apache deployment) and diff the responses to see what I mean.

merlyn has already said that mod_cgi gives you a live connection to the socket, so you just read from STDIN in your CGI program as you would normally. People almost always want decoded form/url-encoded name/value pairs so you will see them use CGI.pm, but there is technically nothing to stop you reading STDIN directly. I use this in CGI programs that support machine/non-human HTTP clients, which POST and PUT XML and YAML in the body of the request, rather than the usual form data. (The contents of the Content-type: header change to "text/xml" or whatever to reflect this, obviously.)

HTH, HAND.