Section 28. A person shall not be excused from attending and testifying before either branch of the general court or before a committee thereof upon a subject referred to such committee on the ground that his testimony or evidence, documentary or otherwise, may tend to criminate him or subject him to a penalty or forfeiture; but he shall not be prosecuted or subjected to a penalty or forfeiture for or on account of any action, matter or thing concerning which he may be required to so testify or produce evidence, except for perjury committed in such testimony.