title: What does "-e" do inside an expression in Perl?
body: <p>I ran into this piece of code: </p>
  
  <pre><code>unless(-e $SNMPWALK)
  {
      print "snmpwalk is not installed!\n";
      exit 3;
  }
  </code></pre>
  
  <p>What is the function of <code>-e</code> in this expression?</p>