It is one correct way to write the datastructure. One of the easiest ways of displaying an entire datastructure is to use Data::Dumper. With the data structure you posted, you would use Data::Dumper like this:

use strict;
use warnings;
use Data::Dumper;

my %t_type = (
    'a' => {
        'chance' => {'low' => 2, 'high' => 12, 'percent' => 50},
        'range' => {'number' => '','disallowed' => ['none','1','2']},
    }
);

print Dumper \%t_type;
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The datastructure you've written is a hash of hashes of hashes, where one of the innermost hash's elements references an array.

What exactly are you trying to do, iterate over the entire structure? Take a slice of a portion of it? What do you need? One place to look for some answers is perldsc: The Perl Datastructure Cookbook; included in the POD of every complete Perl distribution.

You can dereference the deepest level array in scalar context to get its size.