[monolithed]

примерно:

window.onload = function() {
	var element = document.getElementsByTagName('head')[0];
	switch(/\/{2}(\d*\w*)/.exec(document.referrer)[1]) {
		case 'google' :
			element.innerHTML += '<script src=\"google.com\" type=\"text\/javascript\"><\/script>';
		break;
		case 'yandex' :
			element.innerHTML += '<script src=\"yandex.com\" type=\"text\/javascript\"><\/script>';
		break;
		default :
			element.innerHTML += '<script src=\"google.com\" type=\"text\/javascript\"><\/script>';
		break;
	}
};