No person shall be excused from testifying or from producing any book, document, paper or account in any investigation or inquiry by, or hearing before, the Commission or any commissioner, when ordered to do so, upon the ground that the testimony or evidence, book, document, paper or account required of him may tend to incriminate him or subject him to penalty or forfeiture; but no person shall be prosecuted, punished or subjected to any forfeiture or penalty for or on account of any act, transaction, matter or thing concerning which he shall have been compelled under oath to testify or produce documentary evidence. But no person so testifying shall be exempt from prosecution or punishment for any perjury committed by him in his testimony.