title: Perl one-liner with -pl command-line options
body: <p>I'm running</p>
  <pre><code>perl -ple '$_=length' datafile
  </code></pre>
  <p>The datafile contains the following:</p>
  <pre><code>algorithm
  student government
  Fiumichino
  </code></pre>
  <p>The result is that it prints</p>
  <pre><code>9
  18
  10
  </code></pre>
  <p>What do the -p and -l options do? Also, what is <code>$_</code>?</p>