use strict;
use warnings;

use JSON::XS     qw( decode_json );
use Scalar::Util qw( blessed reftype );
use XML::Simple  qw( XMLout );

sub json_tree_to_xml_tree {
    my @to_visit = \( $_[0] );

    while (@to_visit) {
        for my $node (${ pop @to_visit }) {
            if (!defined($node)) {
                die("Can't handle undef\n");
            }

            elsif (JSON::XS::is_bool($node)) {
                # true  => 1
                # false => 0
                $node += 0;
            }

            elsif (blessed($node)) {
                die("Can't handle objects\n");
            }

            elsif (reftype($node)) {
                if (ref($node) eq 'ARRAY') {
                    push @to_visit, map \$_, @$node;
                }

                elsif (ref($node) eq 'HASH') {
                    push @to_visit, map \$_, values %$node;
                }

                else {
                    die("Unexpected reference type\n");
                }
            }
        }
    }
}

my $text = '{"a":"x","b":true}';
my $tree = decode_json($text);
json_tree_to_xml_tree($tree);
print XMLout($tree);
[download]

It (at least) appears a bit simpler with recursion, although it's probably a bit slower.

use strict;
use warnings;

use JSON::XS     qw( decode_json );
use Scalar::Util qw( blessed reftype );
use XML::Simple  qw( XMLout );

sub json_tree_to_xml_tree {
    for my $node (@_) {
        if (!defined($node)) {
            die("Can't handle undef\n");
        }

        elsif (JSON::XS::is_bool($node)) {
            # true  => 1
            # false => 0
            $node += 0;
        }

        elsif (blessed($node)) {
            die("Can't handle objects\n");
        }

        elsif (reftype($node)) {
            if (ref($node) eq 'ARRAY') {
                json_tree_to_xml_tree( @$node );
            }

            elsif (ref($node) eq 'HASH') {
                json_tree_to_xml_tree( values(%$node) );
            }

            else {
                die("Unexpected reference type\n");
            }
        }
    }
}

my $text = '{"a":"x","b":true}';
my $tree = decode_json($text);
json_tree_to_xml_tree($tree);
print XMLout($tree);
[download]