I dont think Sangakkara will open either, but I would open with him if I was a selector.
I saw Perera bowl today, and I think I'd be being kind if I said he wasnt international standard. I dont even think he'd make an Australian first class side. Honestly, if Perera was to be the supersub in the team I named and Sri Lanka happened to bowl second, I probably still wouldnt use him. Ten overs each from Vaas/Dilshan/Maharoof/Murali/Bandara and Arnold to bowl a couple if need be.
Not only would I pick Bandara, but I'd pick Chandana as well if he was in the squad, and leave Maharoof as the supersub. Sri Lanka need to try something different if they are going to challenge this series. They really need to play to their strength, which is spin bowling and natural batsmen. Picking as many spinners as possible, and giving the batsmen freedom to play agressively by having a long batting lineup is the only way Sri Lanka will trouble Australia or South Africa. They cant go into this series with conventional thinking, but Im almost certain they will. Brave tactics like opening the bowling with Dilshan and picking three specialist spinners will give them at least some hope of it strangely pulling off. Gilchrist, Ponting, Smith etc would absolutely hammer Perera, Fernando etc if they played, and trying to get batsmen like Mubarak and Dilshan to play conservatively wont work either - they need to try something different.