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

...

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

...

my $xml = $xs->XMLout($data);
print $xml;

##</code><code>##

<Things>
	<Thing>
		<Item>XXX</Item>
		<Number>9999</Number>
		<Details>
			<name>Detail</name>
			<Color>Blue</Color>
		</Details>
	</Thing>
</Things>

##</code><code>##

<Things>
	<Thing>
		<Item>XXX</Item>
		<Number>9999</Number>
		<Details>
			<Detail>
				<Color>Blue</Color>
			</Detail>
		</Details>
	</Thing>
</Things>