title: How does IPC::Open3 enable autoflush for CHLD_IN
body: <p>The documentation for <a href="http://search.cpan.org/~rjbs/perl-5.24.0/ext/IPC-Open3/lib/IPC/Open3.pm" rel="nofollow noreferrer">IPC::Open3</a> states:</p>
  
  <pre><code>The CHLD_IN will have autoflush turned on
  </code></pre>
  
  <p>But nothing in the <a href="http://cpansearch.perl.org/src/RJBS/perl-5.24.0/ext/IPC-Open3/lib/IPC/Open3.pm" rel="nofollow noreferrer">source code</a> mentions <code>IO::Handle::autoflush</code>. What mechanism does the module use to turn on autoflush for <code>CHLD_IN</code>?</p>