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use XML::Simple;
my $xs = XML::Simple->new( ForceArray => 1, KeepRoot => 1, NoAttr => 1
+ );
...

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

<Things>
    <Thing>
...
        </Details>
    </Thing>
</Things>

<Things>
    <Thing>
...
        </Details>
    </Thing>
</Things>