Hi nos, also see here for an example video:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3042.14

Like Burr says you need to start the first point of the line somewhere in the general area (like within 100 pixels or so) of the perpendicular, and then when you move your mouse over the other curve there will be that "perp/perp" snap available which is the one you want, it's usually just slightly to one side of the single perp snap.

When it kicks in it will read "Perp/Perp" on both sides of the line. That's a special snap that also moves the start point of the line as well.

- Michael