Or using XPaths:

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

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

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

for my $branch_node ($doc->findnodes('/arbre/branche')) {
    $branch_node->setAttribute(type => $types{$branch_node->getAttribu
+te('name')});
}

print $doc->toString;
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