Also see Math::Trig's great_circle_distance function. From the documentation:

# To calculate the distance between London (51.3N 0.5W) 
# and Tokyo (35.7N 139.8E) in kilometers: 

  use Math::Trig qw(great_circle_distance deg2rad);

  # Notice the 90 - latitude: phi zero is at the North Pole.
  @L = (deg2rad(-0.5), deg2rad(90 - 51.3));
  @T = (deg2rad(139.8),deg2rad(90 - 35.7));

  $km = great_circle_distance(@L, @T, 6378);
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