package B;

sub new {
	my $class = shift;	# ignore
	my $type = shift;
	if ($type == 1) {
		return D1->new(@_);
	} elsif ($type == 2) {
		return D2->new(@_);
	} else {
		return undef;
	}	
}

# more as appropriate

package D1;

@D1::ISA = qw( B );

sub new {
	# paranoia check
	# you may not want this and I haven't tested it anyway
	return unless caller eq 'B';
	# more as appropriate
}

package D2;

@D2::ISA = qw( B );

sub new {
	# you may not want this and I haven't tested it anyway
	return unless caller eq 'B';
}