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

my $twig = XML::Twig->parsefile( 'your_file.xml' );

#search for "HostProperties" nodes within the tree. You can
#be more specific about this if you need to. 
foreach my $HP ( $twig->get_xpath('//HostProperties') ) {
    #check if it has a "tag" element with a "mac-address" "name" attribute.
    if ( not $HP->get_xpath('./tag[@name="mac-address"]') ) {
        #insert a new element at the end of the entry. 
        $HP->insert_new_elt( 'last_child', 
                     'tag', { 'name' => 'mac-address' },
                     "DE:AD:BE:EF:F0:FF" );
    }
}

#note - pretty printing helps reading, but might
#cause some problems with certain XML constructs. Shouldn't in your specific example though. 
$twig->set_pretty_print("indented_a");
$twig->print;

#to save it to a file:
open ( my $output, '>', 'processed.xml' ) or die $!;
print {$output} $twig -> sprint;
close ( $output );