By replacing my gluOrtho2D with gluPerspective, what changes shoud I make and where?
You also need to set up the modelview matrix.
After glMatrixMode(GL_MODELVIEW); you need some calls to glScale and glTranslate, to make sure your coordinates are scaled into the -1,1 range.
Anything positioned on z-depth 0 can have the same coordinates as if they were set up in a ortho view.
You need to use different coordinates for your vertices since the coordinate systems are completely different. For a given vertical FOV and aspect ratio, you can calculate the range in X and Y direction at a certain depth Z.
They are only completely different if you choose to make them completely different.
(By for example not setting up the modelview-matrix.)
I'm assuming he want 2d planes in a 3d space, and feels it is convenient to use screen points to position those panes.