There are various Hebrew and Greek words that are translated as perfect in the King James Bible. I find it best to stay with the English and find the unity of thought and meaning there.
Are we sure that the modern translation of the Greek of the word "perfect", in Matthew 5:48 --as "complete" is fully correct or even correct at all?
In context, it would seem that God is telling us to be perfect as that word in English would imply. Now, we know no one can be perfect all the time and therefore be completely sinless like our Lord. But, it is my understanding that we can be perfect for moments and even longer than moments. I believe that.
I suspect revisionists got their hands on the Greek manuscripts or the translations of the Greek manuscripts in past centuries to avoid an accurate rendering of the word "perfect".
In context, it would seem that God is telling us to be perfect, as that word in English would imply. I mean aren't we more "perfect" than "complete" when we are striving against sin and toward righteousness, by hoping not to be paid back on a loan, turning the other cheek when punched meanly, loving our enemy, etc.? Complete can fit but it seems weaker.