Could + perfect infinitive
This form is used for the past ability when the action was not performed;
I could have sent him some money. (But I didn’t send it)
Or when we don’t know whether the action was done or not:
My pen has disappeared. Who could have taken it?
John could have taken it. (He was sitting there.) Doubt
He could have come to see me. But he didn’t come.
He was able to come to see me. That means he came to see me.
She could have sent the book. (She didn’t send that or we don’t know whether she sent or not)
I could have helped him. (But I didn’t do it)