$node->firstChild->setData("blahblah");

##</code><code>##

use strict;
use warnings;
use XML::LibXML 2;

my $doc = 'XML::LibXML'->load_xml(IO => \*DATA);

foreach my $node ( $doc->findnodes('//ReferenceID') )
{
	$node->firstChild->setData("Reference ID type is $node->{referenceIDType}")
		if $node->{referenceIDType} eq 'FXDD';
}

print $doc->toString;
  
__DATA__
<root>
	<ReferenceID referenceIDType="FXDD">FXDDILN233</ReferenceID>
</root>