An example on hooking Pages::saveReady to write a hidden value upon page save. Based on HelloWorld.module from ProcessWire default installation, only slight modifications.

HelloWorld.module

<?php

/**
 * ProcessWire 'Hello world' demonstration module
 *
 * Demonstrates the Module interface and how to add hooks.
 *
 * ProcessWire 2.x
 * Copyright (C) 2010 by Ryan Cramer
 * Licensed under GNU/GPL v2, see LICENSE.TXT
 *
 * http://www.processwire.com
 * http://www.ryancramer.com
 *
 */

class Helloworld extends WireData implements Module {

	/**
	 * getModuleInfo is a module required by all modules to tell ProcessWire about them
	 *
	 * @return array
	 *
	 */
	public static function getModuleInfo() {

		return array(

			// The module'ss title, typically a little more descriptive than the class name
			'title' => 'Hello World',

			// version: major, minor, revision, i.e. 100 = 1.0.0
			'version' => 101,

			// summary is brief description of what this module is
			'summary' => 'An example module used for demonstration purposes. See the /site/modules/Helloworld.module file for details.',

			// Optional URL to more information about the module
			'href' => 'http://www.processwire.com',

			// singular=true: indicates that only one instance of the module is allowed.
			// This is usually what you want for modules that attach hooks.
			'singular' => true,

			// autoload=true: indicates the module should be started with ProcessWire.
			// This is necessary for any modules that attach runtime hooks, otherwise those
			// hooks won't get attached unless some other code calls the module on it's own.
			// Note that autoload modules are almost always also 'singular' (seen above).
			'autoload' => true,
			);
	}

	/**
	 * Initialize the module
	 *
	 * ProcessWire calls this when the module is loaded. For 'autoload' modules, this will be called
	 * when ProcessWire's API is ready. As a result, this is a good place to attach hooks.
	 *
	 */
	public function init() {

		// add a hook after the $pages->saveReady
		$this->pages->addHookAfter('saveReady', $this, 'example1');

	}

	/**
	 * Example1 hooks into the pages->saveReady and writes a value to a hidden field
	 *
	 */
	public function example1($event) {
		// the saved page is given as an argument to the saveReady() method
		// and the first argument is retrieved from the event like this
		$page = $event->arguments[0];

		// do nothing if the page saved has some other template than the one we're aiming for
		if($page->template != 'box-template') return;

		// calculate a value for the hidden field based on some fields given by the user
		$page->hiddenBoxArea = (int)$page->boxWidth * (int)$page->boxHeight;
		
		$this->message("Hello World! You saved {$page->path} with an area of " . $page->hiddenBoxArea);
	}

}