It returns the string (?-xism:[@*+])
- qr/[%s]/ builds a regex that matches strings containing "%" or "s".
- sprintf stringifies the compiled regex to (?-xism:[%s]) to use it as the format pattern.
- The characters are included into the format pattern and the result is returned by sprintf.
So not only do you not end up with a compiled pattern, you waste time compiling a pattern you never use!