I believe I mentioned Math::Trig to you. It is in the standard perl distribution. From the pod:

GREAT CIRCLE DISTANCES
       You can compute spherical distances, called great circle distances, by
       importing the "great_circle_distance" function:

               use Math::Trig 'great_circle_distance'

         $distance = great_circle_distance($theta0, $phi0, $theta1, $phi1, [, $rho]);

       The great circle distance is the shortest distance between two points on a
       sphere.  The distance is in "$rho" units.  The "$rho" is optional, it
       defaults to 1 (the unit sphere), therefore the distance defaults to radi­
       ans.

       If you think geographically the theta are longitudes: zero at the Green­
       which meridian, eastward positive, westward negative--and the phi are lat­
       itudes: zero at the North Pole, northward positive, southward negative.
       NOTE: this formula thinks in mathematics, not geographically: the phi zero
       is at the North Pole, not at the Equator on the west coast of Africa (Bay
       of Guinea).  You need to subtract your geographical coordinates from pi/2
       (also known as 90 degrees).

         $distance = great_circle_distance($lon0, pi/2 - $lat0,
                                           $lon1, pi/2 - $lat1, $rho);

After Compline,
Zaxo