use strict;  #use this!
use warnings;
use XML::Twig;
# use XML::Simple;   NOOOO!


# $localfile= "Test_1.xml";  used DATA instead
my $field = "Nest1";
open my $fout1, '>', "testx.csv" or die "Could not open file!";
my $twig = XML::Twig->new(
                    twig_roots    => {
                                $field => 1,
                                'd1' => 1,
                                'd2'=> 1,
                                },
                    twig_handlers => {
                                'DatatoParse' => \&node,
                                'DatatoParse//*' => \&node1
                                }

                        );
$/='';  ## added this
$twig->parse(<DATA>);   #modified this



sub node {
        my($twig, $el) = @_;
        $twig->purge;
        }

sub node1{
            print  $fout1 "\n", if ($_->tag eq "d1");
            print  $fout1 $_->text, ",", unless ($_->has_children('#ELT'));
            print  $fout1 "\n", if ($_->tag eq "elt");
        }
        
__DATA__
<DatatoParse>
<elt>
....