use Tie::Hash::FixedKeys;

my @keys = qw( a b c );

sub new {
    my $class = shift;
    my %args = @_;

    my $self = {};
    tie %$self, 'Tie::Hash::FixedKeys', @keys;

    foreach my $k ( @keys ) {
        if ( exists $args{$k} ) {
            $self->{$k} = $args->{$k};
        }
    }

    return bless $self, $class;
}