That's not trivial. Defining a constant using the constant pragma means a subroutine with that name is created - a subroutine without a prototype.

Now, it's fairly easy to test if there's a subroutine defined given a name, and it's also fairly easy to see whether it has an empty prototype. But not every function with an empty prototype is a constant.

So, you may have to resort to introspection tools like Devel::Peek. It's Dump function can tell whether a subroutine is a constant - look for the CONST flag. Dump writes to STDERR, so if you want to do this from within your program, you may want to redirect STDERR and capture it yourself.