It works in combination with XML::Simple:

use strict;
use warnings;
use Tie::IxHash;
use XML::Simple;

my %myhash;

tie %myhash, 'Tie::IxHash';
$myhash{foo1} = 'bar1';
$myhash{foo3} = 'bar3';
$myhash{foo2} = 'bar2';
$myhash{foo4} = 'bar4';
$myhash{foo5} = [qw(bar5a bar5b bar5c)];

print XMLout(\%myhash, NoSort => 1);
[download]

And the output:

<opt foo1="bar1" foo3="bar3" foo2="bar2" foo4="bar4">
  <foo5>bar5a</foo5>
  <foo5>bar5b</foo5>
  <foo5>bar5c</foo5>
</opt>
[download]

And without the tie:

<opt foo3="bar3" foo4="bar4" foo2="bar2" foo1="bar1">
  <foo5>bar5a</foo5>
  <foo5>bar5b</foo5>
  <foo5>bar5c</foo5>
</opt>
[download]

Alternatively the data could be written with XML::Twig or other XML modules.