http://active.tutsplus.com/tutorials/effects/create-an-organic-dissolve-with-perlin-noise/

In this tutorial, we’ll be building a class file that creates a stylized mask for any DisplayObject. The mask will be based on Perlin noise, and create an effect of growing holes that progressively “eat away” at the DisplayObject until it is completely obscured. Along the way, we’ll learn some of the more advanced techniques available using BitmapData.