What you have is a pretty specific solution, and as such, is really only useful for you. But, you can indeed trim the code down. Here is one to do that:

use strict;
use warnings;
use XML::Simple;
use Data::Dumper;
 
my $data = XMLin(\*DATA, ForceArray => 1);
 
for my $trans (@{ $data->{trans} }) {
    next unless $trans->{kool} eq 'max';
    $trans->{item} = [$trans->{item}] unless ref $trans->{item} eq 'AR
+RAY';
 
    push @{ $trans->{item} }, {
        maxis => { assignee => ['jill'], status => ['con'] }
    };
}
 
print XMLout($data, KeyAttr => 'id');
 
__DATA__
<opt>
 <trans kool="max">
  <item id="legas">
    <status>contacted</status>
    <assignee>jack</assignee>
  </item>
 </trans>
 <trans kool ="kool">
  <item id="gas">
    <status>contacted</status>
    <assignee>jack</assignee>
  </item>
 </trans>
</opt>
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)