Vasily Heartfelt

function format(string $data): array 
{
    $make = function (array & $result, string $key, int $value) {
        $keys = explode(' ', $key);
        $last = array_pop($keys);

        while ($current = array_shift($keys))
        {
            if (!array_key_exists($current, $result)) {
                $result[$current] = [];
            }

            $result = & $result[$current];
        }

        $result[$last] = + $value;
    };

    $items = array_filter(array_map('trim', explode('<Sum>', $data)));

    $result = [];
    foreach ($items as $item) {
        $item = str_replace(['[', ']'], ['', ' '], $item);
        [$key, $value] = explode('  = ', $item);
        $make($result, $key, +$value);
    }

    return $result;
}