If that were not so, there would be no distinction between hallucinating and perceiving. Hallucination is pathological. Normal perception is always the perception of something that has existence in reality. If we do not assert that the perceptual object also really exists as a perceptible thing, we cannot say that we are perceiving it. No proof of an external world is necessary.
Existence has three modalities.
The first is real existence, existence independent of the human mind and unaffected by it. The second modality is subjective existence. The contents of the human mind, its sensations, perceptions, images, memories, and concepts, have existence in your mind and mine. Since you and I really exist, subjective existence is also one form of real existence. This third mode is the existence of all the objects of the human mind except perceptual objects.