I would probably do something like:

output(\%hash);

sub output {
    my ($hash, @keys) = @_;

    die 'Not a hashref' unless(ref($hash) eq 'HASH');

    foreach my $key (sort keys %$hash) {
        if(ref($hash->{$key}) eq 'HASH') {
            output($hash->{$key}, @keys, $key);
        } else {
            print join(', ', @keys, $key, $hash->{$key}), "\n";
        }
    }
}
[download]

update: added die.