I was a little confused so I must assume that the first instance of <fixe> is not truly your root element. Yet, even with my assumptions, this may help. I added the passing of $it (iteration) and $nn (node name).

Try this:

#!/usr/bin/perl -w -s
use strict;
use XML::LibXML;

my $it = shift || 1;
my $nn = shift || '';

print qq($it\n);

my $parser  = XML::LibXML->new();
my $doc = $parser->parse_file("model.xml") or die "cannot open xml fil
+e: $!";

my $node = $doc->getDocumentElement();
my @dc = $node->getElementsByTagName('fixe');

foreach my $descr (@dc) {
    foreach my $elt ($descr->childNodes()) {
        next unless $elt->nodeType() == 1;      #to avoid text nodes
        if ( $elt->nodeName eq $nn ) {
            $descr->addSibling($elt->cloneNode) foreach ( 1 .. $it );
        }
        else {
            $descr->addSibling($elt);
        }
    }
    my $parent = $descr->parentNode;
    $parent->removeChild($descr);
}

print $doc->toString();
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XML is very particular in that an element is an element, it already exists. Your were on the right track with cloneNode, you just needed to combine that with the appendSibling.

Edit by castaway: Closed small tag in signature