What you have is a php script, this is a perl script

#!/usr/bin/perl
use strict;
use warnings;
use XML::Twig;

open my $fh,'>','sortie.csv' or die "$!";
my $t = XML::Twig->new( 
  twig_handlers => {
    '/xml/element' => sub {
      my $nom    = $_[1]->first_child('nom');
      my $prenom = $_[1]->first_child('prenom');
      $nom    = (defined $nom)    ? $nom->text : '';
      $prenom = (defined $prenom) ? $prenom->text : '';
      print $fh "$nom;$prenom\n";
    }
  },       
);

$t->parsefile('entree.xml');
close $fh;
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