Otherwise,it would be redundant to write the New Testament, since the law and testimony and precepts and commandments of the LORD were (as you interpret "perfect") "complete" right then and there when the Psalms were written, in the OT.
Your post in context on the question at hand.
Perhaps you're not understanding what the Law of the Lord is.
I know Roman Catholics aren't used to Biblical exegesis and this becomes painfully clear in these type of discussions.
And whatever my personal limitations (which you or I may well note) the Law of the Lord is the biggest thing in the Magisterium and happens to be a hot topic within the Catholic Church right now, because the dubious Argentinian Peronist presently resident at Domus Sanctae Martae seems to have forgotten all he ever knew about it.
This lamentable amnesia on his part has drawn this contemporary and ageless response from an esteemed teacher in the Church who is arguably more Catholic than the Pope.
What's the Koine Greek word for "Sheesh"?