If you are determined to do this on your own, eventually you will need to look at the HTTP standard as defined in the relevant RFCs: RFC1945 and RFC2616.

From the information you have given, I would suggest you add an extra newline to the POST header. Also, you will need to send a 'Content-type: multipart/form-data' header and probably need to add some really ugly boundary strings. Yuck.

If I were to do this, I would rush to the LWP-Bundle module, as it handles all of this for you already. See perldoc lwpcook and search for POST.