In Exception§
See primary documentation in context for sub warn
multi warn(*)
Throws a resumable warning exception, which is considered a control exception, and hence is invisible to most normal exception handlers. The outermost control handler will print the warning to $*ERR. After printing the warning, the exception is resumed where it was thrown. To override this behavior, catch the exception in a CONTROL block. A quietly {...} block is the opposite of a try {...} block in that it will suppress any warnings but pass fatal exceptions through.
To simply print to $*ERR, please use note instead. warn should be reserved for use in threatening situations when you don't quite want to throw an exception.
warn "Warning message";