title: What is _ (single underscore) in Perl without $ or @?
body: <p>I have some code like this:</p>
  
  <pre><code>if (!-d _ || !-o _ || ($vncUserDirUnderTmp &amp;&amp; ($mode &amp; 0777) != 0700)) {
      raise Exception("$prog: Wrong type or access mode of $vncUserDir.\n")
  }
  </code></pre>
  
  <p>I could find information about <code>@_</code> and <code>$_</code> in Perl but not about <code>_</code> alone.
  What does it mean in <code>-d _</code> and <code>-o _</code> here?</p>