title: What does !! (double exclamation point) mean?
body: <p>In the code below, from a <a href="http://use.perl.org/use.perl.org/_Alias/journal/39126.html" rel="noreferrer">blog post by Alias</a>, I noticed the use of the double exclamation point <code>!!</code>. I was wondering what it meant and where I could go in the future to find explanations for Perl syntax like this. (Yes, I already searched for <code>!!</code> at <a href="http://perldoc.perl.org/perlsyn.html" rel="noreferrer">perlsyn</a>).</p>
  
  <pre><code>package Foo;
  
  use vars qw{$DEBUG};
  BEGIN {
      $DEBUG = 0 unless defined $DEBUG;
  }
  use constant DEBUG =&gt; !! $DEBUG;
  
  sub foo {
      debug('In sub foo') if DEBUG;
  
      ...
  }
  </code></pre>
  
  <p><strong>UPDATE</strong><BR>
  Thanks for all of your answers.<BR></p>
  
  <p>Here is something else I just found that is related <a href="http://www.perlmonks.org/?node_id=564792" rel="noreferrer">The List Squash Operator <code>x!!</code></a></p>