Make part of a camera viewport invisible/transparent

Hi,
I would like to make a texture or a mesh which would cover part of a camera and that part of the camera would be transparent so other camera would be visible underneath. I was trying to achieve such an effect following the tutorial linked below but it worked only with orthographic camera, not perspective. (also there’s probably a better way to do this) I have unity pro.

so in the end I used this shader. the lightning has to be set to forward rendering for it to work

http://wiki.unity3d.com/index.php?title=DepthMask