A perfect number is a positive integer that equals the sum of its proper divisors (divisors excluding the number itself).
Example: 6 has divisors 1, 2, 3, 6. The proper divisors are 1, 2, 3. Sum: 1 + 2 + 3 = 6. So 6 is perfect.
Given a positive integer num, return true if num is a perfect number, otherwise return false.