Can the side wall of balcony window of brick-concrete structure cast-in-place slab be perforated?

That is usually the railing of the balcony, which mainly plays the role of enclosure. Even if it is removed, it will not adversely affect the structural safety of the balcony. Of course you can open it. But you can't drive too much, otherwise the strength of the fence itself is insufficient, the personal protection function will be discounted, and life can't be ignored.

It should be noted that in rare cases, the balcony side wall will be designed as an upper cantilever beam and also a railing, which can never be removed. Once removed, something will happen.

If the holes within 100 mm are few and scattered, I suggest you look for the property to see the drawings. If it is a brick wall or a concrete wall of about 60 mm, it is generally an ordinary fence. Open it.