Create an iterator, pass over each element looking for 'branche' nodes, and update them when found:

use strict;
use warnings;
use XML::LibXML;
use XML::LibXML::Iterator;

my $parser = XML::LibXML->new();
my $dom    = $parser->load_xml(location => 'input.xml');

my %newtypes = ( courbe => 'conifere', droite => 'resineux' );

my $iter = XML::LibXML::Iterator->new( $dom->documentElement );
while ( $iter->nextNode() ) {
     my $node = $iter->current();
     next unless $node->nodeName eq 'branche';
     $node->setAttribute( 'type',
        $newtypes{$node->getAttribute('name')} );
}

print $dom->toString;
exit 0;

[download]
produces: 
<?xml version="1.0"?>
<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>
    <branche name="droite" type="resineux">
        <description>
            <![CDATA[une branche commune]]>
        </description>
        <feuilles>
            <fleur color="purple" order="1"/>
            <fleur color="green" order="2"/>
        </feuilles>
    </branche>
</arbre>

[download]

In reply to Re: Add attribute to xml with XML::LibXML by Loops
in thread Add attribute to xml with XML::LibXML by Yshyeeni

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