minetest.get_perlin(seeddiff, octaves, persistence, scale)
Constructs a PerlinNoise object with seed derived from the world's seed value. This function must not be called until after the world is finished initializing, so it must be called during some kind of callback rather than at the top level mod initialization code. The PerlinNoise constructor, on the other hand, may be called at any time since it does not depend on the world's seed.
local perlin = minetest.get_perlin(354, 3, 0.7, 100)