You don't provide much information about what you're actually doing to get that result, so I'm going to assume you're drawing a quad with that actual shape. That is, the non-parallell vertical edges is not a cause of perspective, but the actual shape of the quad. If so, this is pretty much what will happen if you're not careful, and isn't really the wrong result. Setting the perspective hint isn't going to help, as OpenGL is already doing what you're telling it to do. It's just that what you're telling it to do is not what you want it to do.

Here's some reading on how to get what I think you want. Clicky.