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