my $default= {
    stackBytes => 4*1024,
    ...,
};

sub getDefault {
    return $default;
}

sub new {
    my $us= shift @_;
    my $opts= shift @_;
    if(  $opts  &&  ! isa($opts,"HASH")  ) {
        $opts= {
            code => $opts,
            args => \@_,
        };
    }
    my $class= ref($us) || $us;
    my $proto= ref($us) ? $us : $class->getDefault();
    my $self= bless { }, $class;
    my @configKeys= keys %{ $class->getDefault() };
    # add error checking to taste here,
    # but this conveys the idea
    @$self{ @configKeys }= @$proto{ @configKeys };
    @$self{ keys %$opts }= values %$opts;
    return $self;
}