AS3 How to make Fog Of War

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AS3

Hello, I have complete a maze game that a character walks arounds, collects things and shoots enemies. The game is produced entirely dynamically. I now want to add a fog of war where the character can only see whats around him on a certain circle radius. I have created the images and background images using sprites/movie clips.

My original idea was to create 5*5 black squares over the entire map and have them all running an eventlistener to see if they are hit or not and change the alpha to 0 if they are hit. But I think that it might bog down the game with too many EVENTs active. Is there a better way to do this?

Thanks.