There are optimizations and there are optimizations. This one isn't worth it, except in special cases. You may have a special case with your debugging example.

The problem with constant is just what you said: maintainability. Over the years, I have seen so many people have problems with the constant module that I consider it a frequent source of bugs. Yes, if you learn all the details of using it in hashes and strings you can make it work, but people "constantly" stumble over it. It's just too error-prone.