Try

perldoc -f -X
[download]

    If any of the file tests (or either the "stat" or "lstat" 
    operator) is 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. (This doesn't work
    with "-t", and you need to remember that lstat() and "-l" leave
    values in the stat structure for the symbolic link, not the real
    file.) (Also, if the stat buffer was filled by an "lstat" call,
    "-T" and "-B" will reset it with the results of "stat _").
    Example:

        print "Can do.\n" if -r $a || -w _ || -x _;

        stat($filename);
        print "Readable\n" if -r _;
        print "Writable\n" if -w _;
        print "Executable\n" if -x _;
        print "Setuid\n" if -u _;
        print "Setgid\n" if -g _;
        print "Sticky\n" if -k _;
        print "Text\n" if -T _;
        print "Binary\n" if -B _;

UPDATE:
"... the point is that it's not obvious ..."

Just showing you where the information is currently found. If you want to make it more obvious you know exactly how to get that ball rolling on your own. Good luck.

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)