Brute-force perfect-number algorithm
Use backticks

Backticks allow you to put the function name between the values instead of before:

isDivisor n a = mod n a == 0


becomes:

isDivisor n a = n `mod` a == 0


Give less general names

The name upUntil tells nothing about perfect numbers, in a tiny script like this this is no problem, but the habit of meningful names will play yo your advantage in  larger scripts

Do not write wrong comments

Comments don't run, but you are not justified to write wrong comments

-- The main prime-finding function:


The function does not find primes.