Your boa is more than likey just showing a normal feeding response to the opening of her cage. Most snakes that are fed in their cages (as mine are) become acustom to accepting food once the cage door has been opened. The snakes simply do not realize that there are other reasons that we need into there cage, such are cleaning or just intacting wiht the snake. My suggesting to you would be this....
open the snake cage and using a snake hook, touch the snake gently for a minute or two. You may get a strike at first aimed toward you or the snake hook, however once your snake figures out that no food is forth coming, it will settle down and allow for the needed cleaning to be done. As an added precaution while cleaning my snakes cages, I will place their hide box over them (backwards) so the opening is facing away from me. While cleaning my snake's cages I am often at awkward angels and in positions that I can not react quickly to if the snake strikes, so i take these added precautions.
A few other things you may want to look at is any smells that you have on you while working with the snakes. Many girls wear perfume of some sort most of the times, many musk type odors can cause an animal to react unpredictably, also if you have been handleing the family pet rabbit, mouse, rat, cat, chicken or any other potential prey item, it too can cause a strike.
The snake may also feel threatened, be in the first stages of shed or have a mite problem that is making it edgey. If you snake has a lot of "huffing and puffing" with hissing and striking then the snake is being defensive. If the snake has alot of tounge flicking and quick strikes then it is a feeding response.
I hope this helps.
Jim Hopkins