title: Which Perl built-ins cannot be overridden in CORE::GLOBAL?
body: <p>The <a href="http://perldoc.perl.org/perlsub.html#Overriding-Built-in-Functions" rel="nofollow noreferrer">Overriding Built-in Functions</a> section of the perlsub documentation provides</p>
  
  <blockquote>
    <p>There is a second method that is sometimes applicable when you wish to override a built-in everywhere, without regard to namespace boundaries. This is achieved by importing a sub into the special namespace <code>CORE::GLOBAL::</code>.</p>
  </blockquote>
  
  <p>and then gives a few examples. At the end, however, is</p>
  
  <blockquote>
    <p>Finally, some built-ins (<em>e.g.</em> <code>exists</code> or <code>grep</code>) can't be overridden.</p>
  </blockquote>
  
  <p>What is the full list?</p>