There are currently two theories (of which I am aware) to explain the use of the perfect subjunctive, in examples from the Latin Vulgate, included in brianpck's answer to Q: Memento quod <subjunctive>. One of these:
"memento quod et ipse servieris in Aegypto et eduxerit te inde Dominus Deus tuus." (Deut 5:15) =
"Remember both that you served as a slave, in Egypt, and that the Lord your God, himself, lead you out of there."
(1) Given that the writer/ speaker is telling a second person what a third party (God) did--this would normally be expressed as indirect speech, "se eduxisse", as opposed to the perfect subjunctive, "eduxerit". Therefore, the perfect subjunctive, here, is an evolved form (4th. Century) of indirect speech, possibly unique to the Vulgate.
Thanks to Figulus: "...if you view attributed causes as an implicit case of indirect speech..."aufugit quod timebat" = "he ran away because, I say, he was afraid" and "aufugit quod timeret" = "he ran away because, he says, he was afraid" (Woodcock #240). Here, the switch to subjunctive, as always, indicates someone's mental state, and it indicates a switch to a point of view other than the speaker's. In "memento quod sumpseris" (9th. Century verse) it could be interpreted to mean "remember that you say you took up". This is obviously not an explicit case of indirect speech. the words "you say" do not appear explicitly, but may be present implicitly, as implied by the subjunctive."
(2) Figulus: "The subjunctive emphasises what is going on in someone's mind, whether true or not."
The corollary: here, a perfect subjunctive describes a completed action in the mind/ thoughts/ memory.
In the above example (Deut. 5:15) there are two perfect subjunctives. It could be argued that each is fulfilling the same role. The speaker tells the second person to remember (memento quod...) a completed action from his memory (Ben Kovitz: "The subjunctive emphasises the person doing the remembering..."). If correct, this must obviate the requirement to express the action of the third party (God) as indirect speech. The grammatical priority goes, not to the third party, but to the recollection from the memory of the second person.
Concluding, the perfect subjunctive, in the Vulgate examples is either a form of indirect speech; a completed thought in the mind; a hybrid of the two.