Soak rice in water for 1 hour, drain well on absorbent paper. Cover mushrooms with water and soak 30 minutes. Discard water and chop mushrooms finely. Combine mushrooms with remaining ingredients and form into balls about the size of a walnut. Roll in the rice, pressing down gently so the rice adheres to the meat. Pour boiling water into lower part of a steamer, place a layer of greaseproof paper on base of top section of steamer. Arrange pearl balls on the paper. Cover with a lid and simmer over a medium gas flame for 30 minutes.