Not easy to figure out exactly what you need, so apologies if I am incorrect.
First, I am uncertain why you require a hash, after all you cannot sort a hash, and it has no order.
The easiest way to get a unique number for each character is to use ord().

my @array = split '', $string;
@hash{@array} = map {ord} @array;
[download]

If you then want to list the key/value pairs, use a 'for' loop with reverse() and sort(). As stated above, it would be best if you looked these up yourself, but to get you started:

for my $key (reverse sort keys %hash) {
   print "$key: $hash{$key}\n"
}
[download]