XML::Simple's interface is pretty odd in that it outputs something different than it takes in by default. (Maybe it could be even a little more simple - and more Perlish...?) But there's an option:

ForceArray => 1 *# in - important*
    This option should be set to '1' to force nested elements to be
    represented as arrays even when there is only one.

...that you can use like so:

use XML::Simple;

my $xml = XMLin(join('',<DATA>),ForceArray => 1);

print XMLout($xml);

__END__
<meta fpi="1234567890">
<isbn>1-234-56789-0</isbn>
<edition>First</edition>
<authorgroup>
    <author>
        <firstname>John</firstname>
        <surname>Smith</surname>
        <authorblurb url="http://www.someurl.com/etc/nothing.php"/>
    </author>
</authorgroup>
<pagenums>384</pagenums>
<pubdate>October 2001</pubdate>
<subjectset>
    <subject>some.lame.subject</subject>
    <subject>another.lame.subject</subject>
</subjectset>
<publisher>
    <publishername>Publisher Inc.</publishername>
    <imprintname>Publisher Inc.</imprintname>
</publisher>
</meta>
[download]

...I think that it outputs something very similar to what you're looking for...

hth