This is actually a great little visual reference for what is going on in Mocha when tracking between the Perspective and Shear options in the tracker. There inevitably will be a bit of confusion as to what these two options will produce and when exactly to use one versus the other. Andreas Urra shows this pretty clearly with this short little video showing what happens with both of these settings in Mocha.
Andreas Urra writes: To the left is a planar surface that is distorted via a shear transformation. To the right is a planar surface that is rotated in 3D space, resulting in a perspective distortion. For the first pass both surfaces were tracked in mocha with the “perspective” option on. For the second pass, both surfaces were tracked only with up to “shear” checked. Notice how shear keeps parallel lines parallel, resulting in a misfit for the right object. Furthermore, in the end, the right shear track has slipped. Whereas the perspective option held the track for both objects perfectly fine. (Tracking was done in Mocha Pro V3. Min % Pixels used was set to 70%)