I found this method of referencing in the HTML::Template module as I was reading through the module source this evening. It was used in quite an interesting manner in the _mtime subroutine which checks for the existence of the requested template file and then obtains the system modification timestamp for the file through stat - This modification timestamp is used in HTML::Template in conjunction with its caching functions. The code from HTML::Template is as follows:

sub _mtime {
    my ($self, $filepath) = @_;
    my $options = $self->{options};

    return(undef) if ($options->{blind_cache});

    # make sure it still exists in the filesystem 
    (-r $filepath) or Carp::confess("HTML::Template : template file $f
+ilepath does not exist or is unreadable.");

    # get the modification time
    return (stat(_))[9];
}
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As you can see, the _ construct is passed to stat following the -r (readable) test on the file, presumably given that this stat information is also cached in a similar manner to file test information.

 

Update

Following through on this at perlfunc:_X, it appears that these file tests obtain the information required to return a positive or negative test result via a system stat call. From perlfunc:_X ...

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. (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.)

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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Ooohhh, Rob no beer function well without!