No person shall be excused from testifying or from producing any papers, books, accounts, or documents in any investigation or inquiry or upon any hearing, when ordered to do so by the Director, Board of Review, or member thereof, or any claims adjudicator, Referee, or a representative of the Director, upon the ground that the testimony or evidence, documentary or otherwise, may tend to incriminate him or subject him to a penalty or forfeiture. But no person shall be prosecuted or subjected to any penalty or forfeiture for or on account of any transaction, matter or thing concerning which he may testify or produce evidence, documentary or otherwise, before any such person or Board of Review: Provided, that such immunity shall extend only to a natural person, who, in obedience to a subpoena, and after claiming his privilege, shall, upon order, give testimony under oath or produce evidence, documentary or otherwise, under oath. No person so testifying shall be exempt from prosecution and punishment for perjury committed in so testifying.