they'll let you in anyway if you are at least dressed "normally". However you will feel a lot better when dressed with a suit, as you might feel a bit underdressed without and the opera is certainly not a place to wear your sneakers.

If you want to avoid carrying a suit (which I understand because it really gets wrinkled all the time and it is a lot of effort to get it straignt again after the flight) you could at least consider wearing suit trousers and a shirt