Sofa cushions are very absorbent so you will most likely not be able to remove all of the perspiration from inside the sofa. However, assuming your sofa can be cleaned with water (check the tag), you can try to remove the outer stain using white vinegar and a clean, dry, white cloth. Blot (do not rub) the stain, working from the outside of the stain to the center, so as not to spread the stain. Follow the vinegar with either upholstery cleaner or mix 1/2 teaspoon liquid dishwashing detergent in a quart of warm water. Beat with a mixer, and clean the furniture using only the suds. Work on a small area at a time, overlapping areas to avoid spotting. Change the rinse water frequently to keep it clean. Use a fan to thoroughly dry the furniture.