This Latin preposition should not be permitted to get too free with the English language. Such convenient abbreviations as m.p.h. and r.p.m. are no doubt with us for good. But generally it is well to confine per to its own language —e.g. per cent, per capita, per stirpes, per contra, and not to prefer per day to a day, or per passenger train to by passenger train, or as per my letter to as I said in my letter.
Even for phrases in which per is linked to a Latin word, there are often English equivalents which serve at least as well, if not better. A letter can equally well be signed AB for CD as CD per pro AB. £100 a year is more natural than £100 per annum. Per se does not ordinarily mean anything more than by itself or in itself.