No person shall be excused from testifying or producing any papers in any inquest proceedings under sections thirty-five to forty, inclusive, on the ground that his testimony may tend to criminate him or subject him to a penalty or forfeiture, but he shall not be prosecuted or be subjected to a penalty or forfeiture for or on account of any action, matter or thing concerning which he may be required so to testify, except for perjury committed in such testimony.
Massachusetts Legal Code