use XML::Simple;
my $xs = XML::Simple->new( ForceArray => 1, KeepRoot => 1, NoAttr => 1, KeyAttr => [] );
my $data = { 'Things' => { 'Thing' => [] } };

...

push @{ $data->{Things}->{Thing} },
	{
	'Item' => $var1,
	'Number' => $var2,
	'Details' => {
			'Detail' => {
					'Color' => $var3
			    	    }
		     },
	};

...

push @{ $data->{Things}->{Thing} },
	{
	'AdditionalInfo' => {
		'Info' => $var4,
		'Stuff' => $var5,
		'MoreDetails' => {
				'ExtraColor' => $var6
			     }
		}
	};