No person shall be excused from testifying or from producing any books, papers, records or memoranda in any investigation or upon any hearing, when ordered to do so by the Department or any officer or employee thereof, upon the ground that the testimony or evidence, documentary or otherwise, may tend to incriminate him or subject him to a criminal penalty, but no person shall be prosecuted or subjected to any criminal penalty for, or on account of, any transaction made or thing concerning which he may testify or produce evidence, documentary or otherwise, before the Department or any officer or employee thereof; provided, that such immunity shall extend only to a natural person who, in obedience to a subpoena, gives testimony under oath or produces evidence, documentary or otherwise, under oath. No person so testifying shall be exempt from prosecution and punishment for perjury committed in so testifying.