Try this:

use Math::Cephes qw/acos/;
for ( -2, -1.1, -1, 0, 1, 1.1, 2 ) {
  print acos($_),$/;
}
[download]

I expect you are getting something similar (i.e. #QNAN), as I am guessing some of the values $d are outside the domain of the "Real" solutions of acos.

-enlil