How to cut out a shape by putting it outside a line

So basically I'm trying to see if i can put a line in the middle of a shape I drew so I can cut the outside portition of the eye cause I know that if I do this righ the shape split by the line will count as two different shapes that I can select independtly from each other and delete one without affecting the other (since i've done this before) but when I try to do it now the shape instead just deletes the section of line tht it overlaps and I want to know if it's a settings I got to turn on or something I messed up (the pupil was drawn with the oval tool and the eye was also drawn with the ovaltool with a bit of help of the line tool used to make the top flat part)