Select two edges as close as possible to the location where they, or their extensions, would cross. The Perpendicular constraint ensures that, where the two edges cross, they (or if curves, their tangents) are at 90° to each other. Using preselect, you can select an edge and a series of other edges to be perpendicular to the first edge before selecting this function.

For Perpendicular constrains on Splines, the curves need to be very close to perpendicular prior to the constraint or the constraint “may not be processed, or may give unexpected results”. In cases like this, look to see if there are extra, unneeded constraints on the spline or its endpoints and remove those if needed, likewise make sure the spline is “Flexible”. If these steps do not help, then try using a tangent constraint instead as tangent constraints are more reliable when working on splines.