my %defaults = ( cat => 1,
		 dog => 2 );
my @allowed = keys(%defaults); # you might allow others in here too.

sub new
{
	my $class = shift;
	my %args = (%defaults, @_);

	my @hash{@allowed} = @args{@allowed};  # ignore arguments
					       # that we're not expecting.
	# do any specific checking here.

	my $self = \%hash;
	return bless ($self, $class);
}