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$output[int($_/3)] += $arr[$_] for 0..@arr-1;

my $k = @arr / 3;
for (0..$k) {
    $output[$_] += $arr[3*$_] + $arr[3*$_+1] + $arr[3*$_ +2];
}

my $k = @arr / 3;
my $b = 0;
...
    $output[$_] = $arr[$b] + $arr[$b+1] + $arr[$b +2];
    $b += 3;
}