title: Inconsistent External Command Output
body: <p>The terminal transcript speaks for itself:</p>
  
  <pre>
  iMac:~$ echo -n a | md5
  0cc175b9c0f1b6a831c399e269772661
  iMac:~$ perl -e 'system "echo -n a | md5"'
  c3392e9373ccca33629d82b17699420f
  </pre>
  
  <p>Note that the  MD5 hash of <code>a</code> is  0cc175b9c0f1b6a831c399e269772661, the first
  result. Why does it turns out to  be different when the same command is called
  by perl?</p>
  
  <hr>
  
  <p>By the way, perl is <em>perl 5, version 12, subversion 4 (v5.12.4) built for darwin-thread-multi-2level</em>. And the system: Mac OS 10.8, Darwin 12.0</p>