Each grand juror shall keep secret whatever he himself or any other grand juror has said, or in what manner he or any other grand juror has voted on a matter before them. Any court may require a grand juror to disclose the testimony of a witness examined before the grand jury, for the purpose of ascertaining whether it is consistent with that given by the witness before the court, or to disclose the testimony given before the grand jury by any person, upon a charge against such person for perjury in giving his testimony or upon trial therefor.