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
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);
The pod goes on with an example.