XML::Simple itself tells you not to use it.

You can use XML::LibXML instead:

#! /usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use XML::LibXML;

my $dom = 'XML::LibXML'->load_xml(location => shift);
for my $main_target ($dom->findnodes('/Targets/main_target')) {
    say "Main Target Name: ", $main_target->findvalue('main_target_nam
+e');
    for my $target ($main_target->findnodes('target')) {
        my $target_name = $target->findvalue('target_name');
        say "\tTarget Name: $target_name";
        say "\t\tSub Target Name: ", $_->findvalue('sub_target_name')
            for $target->findnodes('sub_target');
    }
}
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