Hi,
Thanks for your help.
For your information (or future readers), I had to take two xml files, add attributes to them xml and join them.

For example :

File1 had:

    <?xml version="1.0" encoding="UTF-8"?>
    <arbre>
      <branche name="courbe" >
        <description>
          <![CDATA[une belle branche]]>
        </description>
        <feuilles>
          <fleur color="blue" order="1" />
          <fleur color="white" order="2" />
          <fleur color="yellow" order="3" />
        </feuilles>
      </branche>
      <!--etc...-->
    </arbre>
[download]

File2 had :

    <?xml version="1.0" encoding="UTF-8"?>
    <arbre>
      <branche name="droite" >
        <description>
          <![CDATA[une branche commune]]>
        </description>
        <feuilles>
          <fleur color="purple" order="1" />
          <fleur color="green" order="2" />
        </feuilles>
      </branche>
      <!--etc...-->
    </arbre>
[download]

And the expected result was

    <arbre>
      <branche name="courbe" type="conifere">
        <description>
          <![CDATA[une belle branche]]>
        </description>
        <feuilles>
          <fleur color="blue" order="1" />
          <fleur color="white" order="2" />
          <fleur color="yellow" order="3" />
        </feuilles>
      </branche>
      <!--etc...-->
      <branche name="droite" type="resineux">
        <description>
          <![CDATA[une branche commune]]>
        </description>
        <feuilles>
          <fleur color="purple" order="1" />
          <fleur color="green" order="2" />
        </feuilles>
      </branche>
      <!--etc...-->
    </arbre>
[download]

Here is the code :

sub _mergeXML
{
    my $FILE1   = shift;
    my $FILE2   = shift;
    my $FILEOUT = shift;

    my $parser = XML::LibXML->new();
    my @wanted;

    my $genericDom = $parser->load_xml(location => $FILE1);
    for my $branch_node ($genericDom->findnodes("/arbre/branche"))
    { $branch_node->setAttribute(type => "conifere"); }
    push(@wanted, $genericDom->documentElement->findnodes("./*"));

    my $customDom = $parser->load_xml(location => $FILE2);
    for my $branch_node ($customDom->findnodes("/arbre/branche"))
    { $branch_node->setAttribute(type => "resineux"); }
    push(@wanted, $customDom->documentElement->findnodes("./*"));

    my $new = XML::LibXML::Document->new( '1.0', 'UTF-8' );
    $new->addChild($new->createComment("So, in the end we have a fores
+t..."));

    my $root = XML::LibXML::Element->new( 'arbre' );
    $new->addChild( $root );

    $root->addChild( $_ ) for @wanted;

    my $pp = XML::LibXML::PrettyPrint->new(indent_string => "    ");
    $pp->pretty_print($new);

    open(FILE,">$FILEOUT") or die $!; print FILE $new->toString;close 
+FILE;
}
[download]

Cheers