When brother Yacov (James) refers to the "perfect law of liberty", most modern people think that is something new.
However, in reality, Yacov was quoting from the Psalms. Everyone around him would have immediately knew what he was quoting. Just people today have made him un-Jewish, and don't know what he is saying. Here is what he says:
But whoso looketh into the perfect law of liberty, and continueth [therein], he being not a forgetful hearer, but a doer of the work, this man shall be blessed in his deed.
James 1:25 (KJV, MBM)
He is quoting from Psalms:
The law of YHVH [is] perfect, converting the soul: the testimony of YHVH [is] sure, making wise the simple.
Psalms 19:7 (KJV, MBM)
So shall I keep thy law continually for ever and ever. And I will walk at liberty: for I seek thy precepts.
Psalms 119:44-45 (KJV, MBM)