The underscore filehandle (one of the few features of Perl to which I can claim to have introduced), means "don't actually perform a stat, but use the information cached from the most recent other stat". Well, the docs in perlfunc say it better:

               If any of the file tests (or either the "stat" or
               "lstat" operators) are given the special filehan-
               dle 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" will leave values in the
               stat structure for the symbolic link, not the real
               file.)  (Also, if the stat buffer was filled by a
               "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 _;
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-- Randal L. Schwartz, Perl hacker