title: How to read from a file and do a grep in Perl?
body: <pre><code>open(MR, "&lt;path_to_report");
  $rid;
  </code></pre>
  
  <p>The file might be <strong>very big in size</strong>. It has a unique line inside it in the following format</p>
  
  <pre><code>Root identifier: &lt;some number&gt;
  </code></pre>
  
  <p>for example</p>
  
  <pre><code>Root identifier: 12987
  </code></pre>
  
  <p>I need to extract <code>12987</code> to <code>$rid</code>.</p>
  
  <p>How can I do that in Perl?</p>