Obviously you can do this:

push @{ $data->{Things}->{Thing} },
    {
    'Item' => $var1,
    'Number' => $var2,
    'Details' => {
            'Detail' => {
                    'Color' => $var3
                        }
             },
    'AdditionalInfo' => {
        'Info' => $var4,
        'Stuff' => $var5,
        'MoreDetails' => {
                'ExtraColor' => $var6
                 }
        }
    };
[download]

But probably it is not what you really want. If the real question is "How can I dinamically build the structure that will generate my XML?", the answer is not so simple (aka, I can't make it for you :)):

The right path to enlightment is to understand how complex data structures work in perl (your example id a HoHoHoHoH, not the very simplest) so you can handle them in a better way

As an example, the following code probably do what you expect:

$struc={
        'Item' => $var1,
        'Number' => $var2,
        'Details' => {
                      'Detail' => {
                                   'Color' => $var3
                                  }
                     }
       }
$struc->{AdditionalInfo}={
                          'Info' => $var4,
                          'Stuff' => $var5,
                          'MoreDetails' => {
                                            'ExtraColor' => $var6
                                           }
                         };
push @{ $data->{Things}->{Thing} },$struc;
[download]

Not sure, I can't try it now