Although the word anon sounds old (it is, it is!), it can still be used today to indicate "soon." If you say, "I hear you calling and I will come anon," the person calling you might still be impatient, but they'll be impressed.
The word anon can be found as early as the 11th Century, with the Old English word āne meaning "in one," or "right away." The word is considered archaic, and sounds a little pretentious when used today. It fits much better in older literature such as works by Shakespeare. For example, in Romeo and Juliet, Juliet's nurse calls her just as the girl is bidding goodbye to Romeo, and she keeps putting the Nurse off by saying, "I come anon."