You didn't say what CPL stands for.
So I will assume you mean Circular Polarizing Filter.
I see no wrong with using a cheap filter to learn.

But do not expect a small filter to be satisfactory on a large lens.
It will obstruct the lens and light.
You can use a large filter on a small lens by using by using step down rings.
You should not assume all Sony lenses have the same size filter threads.
They don't.