use warnings;
use strict;
use Text::CSV;
use XML::Twig;

my %data;
my $twig = XML::Twig->new(
	twig_print_outside_roots => 1,
	twig_roots => {
		'HOST_NAME' => sub { $_[1]->set_text($data{host})->print },
		'HOST_IP'   => sub { $_[1]->set_text($data{ip}  )->print },
		'HOST_MAC'  => sub { $_[1]->set_text($data{mac} )->print },
	} );

my $XML = <<'END_XML'; # just for demo
<root>
	<HOST_NAME></HOST_NAME>
	<HOST_IP></HOST_IP>
	<HOST_MAC></HOST_MAC>
</root>
END_XML

my $csv = Text::CSV->new({binary=>1,auto_diag=>2});
while ( my $row = $csv->getline(*DATA) ) {
	@data{"host","ip","mac"} = @$row;
	# here's where you'd need to match up XML file with data row
	$twig->parse($XML);
}
$csv->eof or $csv->error_diag;

__DATA__
hosta,1.1.1.1,00000C123456,
hostb,2.2.2.2,00000C123457,
hostc,3.3.3.3,00000C123458,

##</code><code>##

<root>
	<HOST_NAME>hosta</HOST_NAME>
	<HOST_IP>1.1.1.1</HOST_IP>
	<HOST_MAC>00000C123456</HOST_MAC>
</root>
<root>
	<HOST_NAME>hostb</HOST_NAME>
	<HOST_IP>2.2.2.2</HOST_IP>
	<HOST_MAC>00000C123457</HOST_MAC>
</root>
<root>
	<HOST_NAME>hostc</HOST_NAME>
	<HOST_IP>3.3.3.3</HOST_IP>
	<HOST_MAC>00000C123458</HOST_MAC>
</root>