XML File: Version 1
<root>
        <part>
            <sect>
               <header>
                   1. Purpose and rationale
                   <P>purpose 1</p>
                   <p>Purpose 2</p>
                   <p>purpose 3</p>
                   <l>
                      <li>purpose list 1</li>
                      <li>list2</li>
                    </l>                      
                </header>
            </sect>
         </part>
     </root>

##</code><code>##

 
 <root>
        <part>
            <sect>
               <header>
                   1. Purpose and rationale
                   <P>purpose 1</p>
                   <p>Purpose 2</p>
                   <p>purpose 3</p>
                   <l>
                      <li>purpose list 1</li>
                      <li>list2</li>
                    </l>                      
                  <p>2. Some other heading</p>
                  <p>content 1</p>
                  <p>content 2</p>
                </header>
            </sect>
         </part>
     </root>

##</code><code>##

     my $dom = XML::LibXML->new->parse_file($file);
     my $study_str = 'Purpose and rationale|Study purpose|Study rationale'
    for my $search ('/root/part/sect/header') {
        my $nodeset = $dom->find($search);
    foreach my $node($nodeset -> get_nodelist)
    {
        $node -> string_value;

        if ($node =~ m/$study_str/i)
        {
          my $protocol = $node;
          print $protocol,"\n";
         #go to the next sibling
         while ($node -> { Node }) {
        if ($node -> { Node } -> getNextSibling ) {
         $node -> { Node } = $node -> getNextSibling;
        return $node -> { Node };
        }
        }
        }
}
}