#5.10
               If any of the file tests (or either the "stat" or "lstat"
               operators) are given the special filehandle consisting of a
               solitary underline, then the stat structure of the previous
               file test (or stat operator) is used, saving a system call.

# ... special cases + example code ...

# BUT ALSO
               As of Perl 5.9.1, as a form of purely syntactic sugar, you can
               stack file test operators, in a way that "-f -w -x $file" is
               equivalent to "-x $file && -w _ && -f _