"perlin(x), perlin(x, y), perlin(x, y, z), perlin(x, y, z, seed)
returns a noise value from 0.0 to 1.0 (roughly) for 1, 2 or 3 dimensional coordinate. The default seed it samples from is 0, but seed can be specified as a 4th argument as well. In case you need 1D or 2D noise values with custom seed, use null for z, or y and z arguments respectively."
Latter emphasis added. The last sentence should be changed to:
"In case you need 2D or 1D noise values with custom seed, use null for z, or y and z arguments respectively."