The ViewCube navigation tool (common to all Autodesk 3D software) can be also used to switch perspective projection on and off.
Right-click on the ViewCube and choose one of the options in the context menu:
- Perspective with Ortho Faces
The first two options can be used to switch the perspective projection off and on. Remember that in the perspective projection, the closer you are to the displayed object, the more distorted will it appear.
The third option (with rather cryptic name) is just a "conditional perspective view". It switches the current view to perspective projection unless the model is being viewed from one of the face views: top, bottom, front, back, left, or right.
The ViewCube perspective setting is stored separately. Even if you disable perspective using the PERSPECTIVE variable, hitting the Home icon in ViewCube will restore the original ViewCube-setting or perspective projection.