In your example, $h is not a tied hash. $h is a blessed reference to an object that is used to store data for the tied hash. The tied hash is %new_hash.

For a better example than the one you are presenting:

use Scalar::Util qw(reftype);

tie(%hash, "ScalarHash");
function_that_expects_a_hash_reference(\%hash);

sub function_that_expects_a_hash_reference
{
    my($hash_ref) = @_;
    reftype($hash_ref) eq 'HASH'
        or die "CALLED WITHOUT A HASH REFERENCE!\n";
    ... $hash_ref->{...} ...
}

package ScalarHash;

sub TIEHASH { bless \(my $foo), $_[0] }
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As I pointed out in the node that you were responding to, you are confusing Perl object interfaces (the ability to use a scalar as if it were a hash reference), and Perl blessed reference methods (the ability to use a particular method with an object).