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.

How do I access the size of the 'disallowed' array?

my $length = @{$t_type{'a'}{'range'}{'disallowed'}};

print "Length: $length\n";
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What is the best way to foreach this array?

print "$_\n" foreach (@{$t_type{'a'}{'range'}{'disallowed'}})
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Just adding a few things to the replies by davido and grantm:

HTH

Update: fixed typo (there is no 'foreach' in English!)(thanks revdiablo!).

With reference to accessing the data, the following construct is also valid:

$#{ expr_returning_arrayref }
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The expr_returning_arrayref is useful in many situations. In an OO application, you might access your array size like this (somewhat contrived for the purpose of example):

my $size = @{ $self->get_disallowed_array };
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Of course, the accessor method would probably use something along the lines of previous replies.

With reference to everything else, excellent replies above.