A quick search doesn’t turn up much.
The Catechism does not discuss this question.
It is safe to presume that a person who refuses to abandon the Faith, knowing he will be killed as a result, and does this out of love, has by that fact made an act of perfect contrition.
I can think of one example off the top of my head. There is a group of recognized martyrs who were killed by Protestants in Belgium or Holland, I believe. One of them was a priest who was a notorious womanizer, and he was confronted with this fact, and mocked by his persecutors. I.e., they tried to shame him, hoping to break his resolve.
He replied: “A fornicator I always was; a heretic I never was.” He is honored as a martyr with all the others of his group. If I can find an account of this incident, I will post it. Of course, there is no way to know whether he might have made a good confession since his last mortal sin.
I would be very surprised to find that there has ever been a magisterial document dealing with this precise question. The answer would be found in the consensus of moral theologians.
The fact that martyrdom obviates the need for Baptism strongly suggests that it obviates the necessity of sacramental absolution as well.
The sacraments were given to us as the ordinary means of grace. They were not created by God in order to close off the possibility of the action of grace by other channels.
The fact that martyrdom obviates the need for Baptism strongly suggests that it obviates the necessity of sacramental absolution as well.
That makes sense.