sub valid_params {
    my %valid = (
        moo => 1,
        woof => 1,
        meow => 1,
    );
    return %valid;
}
sub _validate_params {
    my %p = @_;
    
    my %valid = valid_params();

    for (keys %p){
        die "$_ isn't a valid param" if ! exists $valid{$_};
    }
    if (exists $p{moo} && ref $p{moo} ne 'HASH'){
        die "param 'moo' must be a hash reference";
    }
}