Yes and no. Flix does not support top-level constants. You can, however, declare a pure function which takes zero arguments:
def pi(): Float64 = 3.14f64The Flix compiler will inline such constants.
If you have an expensive computation that you want to perform once, you should compute it where needed and explicitly pass it around. Flix does not support top-level constants because they violate the principle that no code should be executed before main.