#!/usr/bin/perl -w

use strict;
use XML::Twig;

my $xml = XML::Twig->new(
                         pretty_print => 'indented',
                        );
$xml->parse(\*DATA);

# get all parents.
my @parents = do {
    my %name = map {
        my $p = $_->parent();
        "$p" => $p;
    } $xml->root()->get_xpath('//name');
    values %name;
};

foreach my $parent (@parents)
{
    my @name = $parent->get_xpath('name');
    my @names = map { $_->cut(); $_->text() } @name;
    
    my $names = XML::Twig::Elt->new('names', join ',', @names);
    $names->paste($parent);
}

$xml->print;

__END__
<doc>
 <set>
  <name>abc</name>
  <name>def</name>
  <name>ghi blah</name>
 </set>
 <set>
  <name>jkl</name>
  <name>mno</name>
  <name>pqr blah</name>
 </set>
 <set>
  <name>abc</name>
  <name>def</name>
  <name>ghi blah</name>
 </set>
</doc>
[download]

<doc>
  <set>
    <names>abc,def,ghi blah</names>
  </set>
  <set>
    <names>jkl,mno,pqr blah</names>
  </set>
  <set>
    <names>abc,def,ghi blah</names>
  </set>
</doc>
[download]