I don't know the module, but some basic trig knowledge can help. In a right triangle, there is a simple relationship between angles and sides. If you have all three sides (SSS), then the following relationship holds true. Here's a diagram that demonstrates the SOH CAH TOA acronym. (sin = opposite/hypotenuse, cos = adjacant/hypotenuse, tan = opposite/adjacant). You could take this knowledge and create a routine that takes the sides in a specific order, calculates the sine values for the two unknown angles and then uses Math::Trig::asin() to get the angles. Seems quite long winded though :)

  A
  |\         sin A = a/b {(b is hypotenuse 4)
  | \        sin A = 3/5
  |  \
4 |   \ 5    sin C = c/b
  |    \     sin C = 4/5
  |90   \
  -------|
 B   3    C

Update: Not to mention that you only need to calculate one of the unknown angles to get all three. Since you have the 90 degrees, calculate one, then knowing that all 3 angles of a triangle have a sum of 180, you just subtract the two known angles from 180 and there's your third.