It appears to me that forms in Unity 3D are only handled via CGI/Perl. Is there more?

How you process your forms at the server side is not up to unity. Don't know what you're looking for but if you want parameters to be passed to unity in web site there are several options. On way is to make the html page communicate to Unity directly:

<script type="text/javascript" language="javascript">
<!--
// BLOCK OF VALUES GENERATED BY SERVERSIDE SCRIPT
var value1 = "Jaap Kreijkamp";
var value2 = 38;
// END OF BLOCK OF VALUES
function GetFormValues()
{
    GetUnity().SendMessage("MyFormProcessor", "SetValue1", value1);
    GetUnity().SendMessage("MyFormProcessor", "SetValue2", value2);
    // etc...
}
-->
</script>

or the other way round, for example you let the server generate a page with several javascript variables set on the returned page depending on the form input. When Unity wants to get the values it can call:

Application.ExternalCall("GetFormValues");

As you can see, it's easy to use dynamic pages as well (where the form results don't have to pass through to the server but can be send to unity directly when pressing form submit button meaning you don't have to leave the page (and stop the unity plugin).