title: Print unique lines of a 10GB file
body: <p>I have a 10GB file with 200 million lines. I need to get unique lines of this file. </p>
  
  <p>My code:</p>
  
  <pre><code> while(&lt;&gt;) {
       chomp;
       $tmp{$_}=1;
   }
   #print...
  </code></pre>
  
  <p>I only have 2GB memory. How can I solve this problem?</p>