I think your best bet is to use this function called segment_intersects_sphere

https://docs.godotengine.org/en/stable/classes/class_geometry.html#class-geometry-method-segment-intersects-sphere

First you need 2 Vector3 3D positions for the line start and end. You have the mouse positions in screen space, but you want the world positions in the scene. You would use the function project_position with the depth (z) value is the distance to the plane your objects are on. In most cases this would be object.translation.z - camera.translation.z. If the camera or the object plane never move, then this can be a constant float value that you set on ready.

https://docs.godotengine.org/en/stable/classes/class_camera.html#class-camera-method-project-position

From there you call it twice, with each of the mouse positions (start and end) which give you the start and end points in 3D world space. From there you can use the segment intersects sphere function I mentioned above. You use the two 3D points to define the segment. Then you have a for loop which checks each of the fruit objects with a rough sphere to approximate their hit box. This should give you at least the first point of intersection, or maybe the exit point as well, you'll have to check what's in the array. You can also use segment_intersects_convex with a more complex collision shape, but this may be more complicated than needed, as the slice will be fast and does not need to be super accurate. You also know the angle of the slice (because you have the mouse points) se even just knowing the object is touching is enough to do a convincing slice. I hope that makes sense.