Good catch. What follows is a more detailed explanation.

The original code

$total += @$score;
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adds an array to $total. What's the value of an array? Well, arrays don't naturally have a value. It turns out it's very convenient for arrays to return the number of elements it contains as its value. It allows us to replace calls to (non-existent) function array_length(@array) with just @array, a very common operation.

If you want to sum the values of the array's elements, you'll have to sum the values of the array's elements. I showed two ways of doing that in my earlier post.