Further to what Corion said, you actually created a hash with a single key. The key was a hash reference and its value undef. The braces {...} create an anonymous hash and return a reference to it. You could have done this:

my $Cls = {'P','a','N','b','U','c'};
print %$Cls,": \n\n";
while ( my ($k, $v) = each(%$Cls) ) {
    print "$k => $v\n";
}
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Note in the print statement I changed the '.' to a ','. The dot operator forces the hash into scalar context, which is rairly useful.