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<header>
   <id x_id="1">
      <a></a>
...
      <c>NA</c>
   </id>
</header>

use strict;
use warnings;
use XML::LibXML;
...
my($object0) = $doc->findnodes("/header/id/c/text()");
my $text0 = XML::LibXML::Text->new($node_c);
$object0->replaceNode($text0);