Iterated digits squaring

func digit_square_sum_iter(n) is cached {

    if ((n == 1) || (n == 89)) {
        return n
    }

    __FUNC__(n.digits.sum { .sqr })
}

say (1..1e6 -> count_by { digit_square_sum_iter(_) == 89 })

Output:

856929

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