in reply to XML::Simple Meets Complex Hash Structure

Switching to a less simple, but simplifying module such as XML::Twig may help. Consider:

use strict;
use warnings;
use XML::Twig;

my $twig = XML::Twig->new (
        twig_handlers => {
            Preroll => \&handleNode,
            Midroll => \&handleNode,
            Postroll => \&handleNode,
        }
    );

$twig->parse (do {local $/; <DATA>});

sub handleNode {
    my ($tag, $elt) = @_;
    my $creative = $elt->first_child ('Creative');
    
    print "Creative: ", $creative->text (), "\n";
}


__DATA__

[download]
<AdXML>
<Preroll>
<Creative>Preroll_30sec</Creative>
<CompanionId>N/A</CompanionId>
<Impression>TBD</Impression>
<Completion>http://192.168.0.1:80/foo/bar</Completion>
<TrackingId>null:414</TrackingId>
<Length>4</Length>
</Preroll>
<Postroll>
<Creative>Postroll_60sec</Creative>
<CompanionId>N/A</CompanionId>
<Impression>TBD</Impression>
<Completion>http://192.168.0.1:80/foo/bar</Completion>
<TrackingId>null:418</TrackingId>
<Length>6</Length>
</Postroll>
<Midroll>
<Creative>Midroll_45sec_3</Creative>
<CompanionId>N/A</CompanionId>
<Impression>TBD</Impression>
<Completion>http://192.168.0.1:80/foo/bar</Completion>
<TrackingId>null:417</TrackingId>
<Length>5</Length>
</Midroll>
<Midroll>
<Creative>Midroll_45sec_1</Creative>
<CompanionId>N/A</CompanionId>
<Impression>TBD</Impression>
<Completion>http://192.168.0.1:80/foo/bar</Completion>
<TrackingId>null:415</TrackingId>
<Length>5</Length>
</Midroll>
<Midroll>
<Creative>Midroll_45sec_2</Creative>
<CompanionId>N/A</CompanionId>
<Impression>TBD</Impression>
<Completion>http://192.168.0.1:80/foo/bar</Completion>
<TrackingId>null:416</TrackingId>
<Length>5</Length>
</Midroll>
</AdXML>

[download]

Prints:

Creative: Preroll_30sec
Creative: Postroll_60sec
Creative: Midroll_45sec_3
Creative: Midroll_45sec_1
Creative: Midroll_45sec_2

[download]
Perl is environmentally friendly - it saves trees

In Section Seekers of Perl Wisdom