#!/usr/bin/perl

use strict;
use warnings;

use XML::Rules;

my @rules = (
  _default => sub {$_[0] => $_[1]->{_content}},
  query =>    sub { print "$_[1]->{name}: $_[1]->{topN}\n" },
);
my $r = XML::Rules->new(rules => \@rules);
$r->parse(\*DATA);

__END__
<?xml version='1.0'?>
<queries>
    <query>
        <name>topHosts</name>
        <layer>LINK</layer>
        <topN>20</topN>
        <datatype>topHosts</datatype>
        <filter></filter>
    </query>
    <query>
        <name>topHosts</name>
        <layer>LINK</layer>
        <topN>120</topN>
        <datatype>topHosts</datatype>
        <filter></filter>
    </query>
</queries>

##</code><code>##

use strict;
use warnings;

use XML::Rules;
use Data::Dumper qw(Dumper);

my @rules = (
  _default => 'content',
  # Default filter to 'None' (v5.10 syntax here)
  filter   => sub { $_[1]{_content} //= 'None'; $_[0] => $_[1]{_content} },
  query    =>    'no content array',
  queries  => 'pass no content',
);
my $r = XML::Rules->new(rules => \@rules);
my $data = $r->parse(\*DATA); # Assume same data as above
print Dumper $data;

# Output
$VAR1 = {
          'query' => [
                     {
                       'topN' => '20',
                       'layer' => 'LINK',
                       'filter' => 'None',
                       'name' => 'topHosts',
                       'datatype' => 'topHosts'
                     },
                     {
                       'topN' => '120',
                       'layer' => 'LINK',
                       'filter' => 'None',
                       'name' => 'topHosts',
                       'datatype' => 'topHosts'
                     }
                   ]
        };