You have to `POST` the data somehow, or send it via `GET` (not good if you have a lot of data, and not really the intended purpose of `GET` anyway). 

It is really hard to tell exactly what your problem is though. The options that come to my mind are:

 1. Use the [AJAX API][1] (even if the request isn't my by Javascript).
 2. Submit to the page you've already created.

Submitting to a different page is tricky if you need WordPress functions, and probably not worth the effort.

I'd use option #2. You can process the form in the same block of code that creates the plugin page, but if you need to redirect ([PRG][2]) you will need to hook the processing function into an early hook. Look at the [`wp-admin/admin.php`][3] file. Starting about halfway down you start to get a bunch of hooks beginning with the very general `admin_init` but also including some very specific hooks like `'load-' . $page_hook` and `'load-' . $plugin_page`.


  [1]: http://codex.wordpress.org/AJAX_in_Plugins
  [2]: http://en.wikipedia.org/wiki/Post/Redirect/Get
  [3]: http://core.trac.wordpress.org/browser/trunk/src/wp-admin/admin.php