Well, the perfect is usually used to look at a past action as a whole (historic perfect) ... and it works that way with sum as with any other verb ...
an example of the pluperfect:
From Ovid, met. 1,87f.:
sic, modo quae fuerat rudis et sine imagine, tellus
induit ignotas hominum conversa figuras.
Thus, the earth, that had a moment earlier been unshaped and without image, was changed and put on the unknown shapes of humans.
When I made I last post, I was referring specifically to the verb sum, lol.That's interesting. I've never really seen those forms used by themselves as a main verb before.