No person shall be excused from testifying or producing any book or document in any investigation or hearing, when ordered so to do by the members of the commission holding the same, upon the ground that testimony or documentary evidence required of him may tend to incriminate or subject him to penalty or forfeiture, but no person shall be prosecuted, punished or subjected to any penalty or forfeiture on account of any matter or thing concerning which he shall, under oath, have testified or produced documentary evidence, except that he shall not be exempt from prosecution or punishment for any perjury committed by him in his testimony.
N.J.S. § 5:8-15