use strict;
use warnings;
use XML::TreeBuilder;

my $xmlDoc = <<XML;
<doc>
...
</doc>
XML

my $root = XML::TreeBuilder->new ();

$root->parse ($xmlDoc);

my @authors = $root->look_down (_tag => 'Author');

for my $authorElt (@authors) {
    my ($lastName) = $authorElt->look_down (_tag => 'LastName');
    my ($foreName) = $authorElt->look_down (_tag => 'ForeName');

    next unless defined $lastName and defined $foreName;

    print $lastName->as_text (), ', ', $foreName->as_text (), "\n";
}

##</code><code>##

van Beilen, J B
Penninga, D
Witholt, B
Wilde, A
Reaves, B
Banting, G