I assume that Kant says that the categorical imperative exhausts all morality? I'm guessing so, both perfect and imperfect duties.
There's, I think, a particular conclusion that appears a little esoteric.
We should tend to deceive ourselves, on the basis of imperfect duties, into something that would otherwise deny our perfect duties. So, if we forget that it's a lie, we ought try to lie to save a child from the Nazis, because it is an imperfect duty to deceive ourselves about the consequences of not lying if it saves an innocent's life. As we have no "perfect duty" against self deception.
Does that sound about right, and, if so, does that change how to conceive of (intuitively model?) the difference between perfect and imperfect duties? I would guess that we just live with imperfect duties, no perfect duties, unless there's a time at which we can no longer tell ourselves that something is not a perfect duty, that the lie is not a lie.
The peculiar conclusion, as it seems to me, is that perfect duties exist if and only if we are aware of them. Imperfect duties, on the other hand, might also change with self deception. But that suggests there's no foundation of morality at all! There must be a perfect duty against self deception about imperfect duties. Is there?
The problem, as I see it, with that, is that it seems to say that we / God never really have moral reason to condemn someone (perfect duties), practically (surely no-one has to be fully conscious that they are lying in order to lie?), except for a lack of conformity; only not reward them (imperfect duty) for not trying hard enough.
That will be completely impossible to agree with, if you think we don't deceive ourselves at will. But if self deception is possible I see no reason to think that it is impossible at certain times.