In the BDB entry for הוא http://biblehub.com/hebrew/1931.htm, the term "Qr perpetuum" appears, but I have no clue what it means. From logical deduction, I can conclude it means something like "from here on out", or "at all times", but google wasn't helpful.
The "Masoretic Text" of the Hebrew Bible includes as part of its "mark up" a set of marginal readings known to the Torah scholars who transmitted the text in that form. These variants are known as "ketiv and qere" readings (see also M. Graves, "The Origins of Ketiv-Qere Readings"):
- ketiv = what is "written" (ketiv), and appears in the text itself;
- qere = what is "read" (qere), and this is the marginal reading.
The qere "Perpetuum" are those readings where the Masoretes (these "Torah scholars") have a ketiv combined with the vocalization of the qere, but without an accompanying marginal annotation. The parade example is the 3rd person singular independent pronoun in the Pentateuch: very many times you get h-w-ʾ (hûʾ, masculine) where you expect h-y-ʾ (hîʾ, feminine). In this case the Masoretic text has the vowel marking of the latter, with the consonants of the former, leading to the anomalous: הִוא. So in every case where this appears the ketiv is seen, but the (unmarked) qere is pronounced (hîʾ) - in "perpetuity", as it were. The first time this occurs is (I think) in Genesis 3:12, the last time in Deut 30:13, altogether 128 occurrences.
- Gesenius-Kautzsch-Cowley, Gesenius' Hebrew Grammar (2nd edn; Clarendon Press, 1910), at § 17c, p. 66 for technical explanation and further examples.
- G. Khan, "Ketiv and Qere", in Encyclopedia of Hebrew Language and Linguistics, ed. by G. Khan (Brill, 2013), vol. 2, pp. 463-468 (qere perpetuum briefly discussed on p. 464a).