update: meh, I completely read right past where you said you wanted to process in XML. I digress.../update

As Corion mentioned, this is JSON, and is exceptionally common for web API returns. Here's how you can import it into a Perl structure and get at it.

use warnings;
use strict;

use Data::Dumper;
use JSON::XS;

my $json;
{
    local $/;
    $json = <DATA>;
}

my $perl = decode_json $json;

for my $href (@$perl){
    print "vendor: $href->{Header}{VendorName}\n";
    for my $item (@{ $href->{Items} }){
        print "\track num: $item->{RackNumber}\n";
    }
}

__DATA__
...paste the JSON string here
[download]

output:

vendor: Some Business
        rack num: 3127824
        rack num: 3127825
vendor: Some Business
        rack num: 3127828
        rack num: 3128353
        rack num: 3128796
        rack num: 3128797
[download]

Now, a couple of notes. I'm going to rework it with your actual code, and explain what's happening. You don't need the special $/ block in your case, nor the __DATA__ section:

use warnings;
use strict;
use HTTP::Request::Common qw(GET);
use JSON::XS;
use LWP::UserAgent;
 
my $ua = LWP::UserAgent->new;
my $endpoint = 'http://blah';

my $req = HTTP::Request->new('GET');
$req->url($endpoint);

my $resp = $ua->request($req);
my $json = $resp->content;

my $perl = decode_json $json;

# it's an array of hashes of hashes (AoHoH). We need to
# access it like this:

for my $href (@$perl){
    # grabbed each hash reference from the top-level array
    # ...now, print the vendor's name

    print "vendor: $href->{Header}{VendorName}\n";

    for my $item (@{ $href->{Items} }){
        # each top level href has a Header href, and many
        # Item hrefs inside of an array ref. Here, we're
        # looping over each Item, and doing something

        print "\track num: $item->{RackNumber}\n";
    }
}
[download]