This could allow special numeric characters such as ¾ to be removed. See isnumeric documentation.

In Julia this is a one liner. 7kyu would be a better choice.

There is a equation for the sum of the squares. Finding the root of this equation would find you the answer, but you would need to find the root of a cubic equation. m(m+1)(2m+1)/6 = sum of squares (1^2+2^2+3^2....m^2)

isletter will also find letters like alpha and beta as true.

Generic solution!

With the Julia version I get unordered tests arrays:
ary = [1, -4, -1, 1, -5, 5, -4, -2] and val = 3

8balls is just a little shorter ;)

This is not the best practice, because Julia has a partialsort function, which is more efficient.

Splitting twice?

Is it the most efficient method? At worst you need to sum the complete array length(arr) times. In principle you only need to take the previous answer and add or substract a single item with each step.