title: how do I check a format of input file
body: <p>I have a file:</p>
  
  <pre><code>aaaa ddd
  aaaa ddd
  </code></pre>
  
  <p>I want to check the format: <code>aaaa(string) dddd(string)</code>
  there is only one space between aaaa and dddd</p>
  
  <p>for example:</p>
  
  <pre><code> aaaa ddd
  aaaa ddd
  </code></pre>
  
  <p>or </p>
  
  <pre><code>aaaaddd
  aaaa ddd
  </code></pre>
  
  <p>both are wrong.
  the size of string is not considerable...
  how do I write it in perl?</p>