If you want a simple value to be processed as an element, make it an array reference.

#!/usr/bin/perl

use warnings;
use strict;
use XML::Simple;

my $data = {
        e => ['element value'],
        a => 'attribute value'
};

print XMLout($data, RootName => 'root');

__DATA__
<root a="attribute value">
  <e>element value</e>
</root>
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