Line1 is the given line and C is the point external to it.
Build the line Line1, using Line and the point C using Point.
Build a circle Circle1, centered in C, using Circle with center point, to intersect Line1 in two points E and F.
Build two other circles Circle2 and Circle3, centered in E and F, respectively, building a segment-radius and using Circle with pt two pts.
These two circles intersect in P.
Now, build the line Line2, using Line, and joining P with C.
This is the desired perpendicular.
Infact, should you joined the points P, E, O, F, you'd obtain a rhomb and PO, EF would be the diagonals of the rhomb itself. As the diagonals of a rhomb are perpendicular, PO is perpendicular to r.

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