If you make an Act of Perfect Contrition then you are no longer in Mortal Sin. So -- no exceptions.
Right. I do want to add, however, that Trent also states that a perfect act of contrition must be accompanied by the intention to go to Confession in order to restore to justification (something that's often missed by people who discuss perfect contrition).
But, as you wrote, there are no exceptions to the rule that one cannot go to Heaven if he dies in a state or mortal sin. Perfect contrition along with the intention to go to Confession ("in due time") would take the soul out of a state of moral sin. Trent adds "in due time" because it's not necessary to run out and wake up a priest at 3AM to go to Confession in order for there to be a solid intention to go to Confession.