Theory and practice of the confessional by Caspar Erich Schieler, Richard Frederick Clarke
From what has been said it follows that a penitent incurs venial sin by a lie told in confession when
(1) he accuses himself falsely of a venial sin or denies having committed a venial sin; except where this venial sin forms the sole matter of confession, for then he would sin mortally, not on account of the lie, but on account of the grave irreverence done to the Sacrament in offering to the priest insufficient matter, for sins falsely stated can never be matter for absolution.
(2) Moreover, it is only a venial sin if the penitent denies having committed a mortal sin which he is not bound hic et nunc to disclose, either because he has already revealed it in a valid confession or because he has pressing reasons for not disclosing it hic et nunc. Indeed it is possible that there is no sin at all when a penitent makes use of mental reservation. The confessor has no right to put questions which have no connection with the materia necessaria, and the penitent is not bound to answer such questions; to avoid a lie he may use a mental reservation by choosing an ambiguous expression which contains the truth, leaving the confessor to judge for himself. If, on the contrary, the priest has a right to inquire of the penitent whether he has committed some grave sin which has been already confessed, and the penitent denies the charge, he would sin mortally.
(3) If the penitent is questioned by the priest as to his home, his condition, or his relatives or friends, and answers not according to the truth, knowing that these questions have no bearing on the nature of his sins, such untruths are only venial; for if a lie told in confession with respect to venial sins, although these may be matter of confession, be only a venial sin, a lie with respect to other things which have no connection with the accusation of the sins is still less likely to be mortal.