I know a couple people who overdose on the perfume. One of them said that she couldn't even smell it on herself anymore, she'd become so desensitized to it. When she was asked to stop wearing it (our boss is really sensitive to smells), she still smelled perfumy for a while after that, because scents had just permeated all her clothing, coat, etc.. Plus she used other scented products, like lotion and shampoo, that she didn't even really think of as "scented" by that point. I think a lot of people just kind of slowly build up more and more scent, not even realizing it, until it's finally brought to their attention that it's bothering people.
I think at this point the OP has a few options. I would definitely not advocate stopping my own perfume use while Carla continued to get away with hers--I understand the boss's position of trying to placate the squeaky wheel, but I think that's really not a good way to do things in this case.
I actually think asking Carla to stop wearing perfume, and in exchange the OP would too, is a great suggestion, done sincerely--if Carla refuses, or agrees but then breaks it, the OP can say she tried in good faith to resolve it on an individual level and it didn't work.
I think the OP could also wait a few days and then ask the boss what his plans are with this issue, because she's not prepared to drop it--in other words, she's going to be a (politely) squeaky wheel, too, and force him to step up and decide something.
The OP might also check with HR as well, just in a general way, to see what company policy is on this issue and if there's any precedent for it. That way if her boss refuses to make a decision, or decides something the OP thinks is unfair (like, Carla can have perfume, but YOU can't), she'll be prepared to take the next step. Maybe the boss would actually prefer to have someone from "up above" make the decision, that way he could have an out with Carla.