sub new {
    my($proto, $fields) = @_;
    my($class) = ref $proto || $proto;

    $fields = {} unless defined $fields;

    # make a copy of $fields.
    bless {%$fields}, $class;
}

##</code><code>##


my $singleton;

sub new {
    my($proto, $fields) = @_;
    my($class) = ref $proto || $proto;

    if ( ! defined $singleton ) {
        $fields = {} unless defined $fields;

        # make a copy of $fields.
        $singleton = bless {%$fields}, $class;
    }

    return $singleton;
}

##</code><code>##

my $c1 = C->new( foo => 'bar' );
my $c2 = C->new( foo => 12345 );