package Simple;
use Class::MethodMaker
  get_set => [qw(a b)],
  new_with_init => 'new';
sub init  {
  my $self         = shift;
  my %in_args      = @_;
  my %default_args = ( b => "default");
  my %final_args   = (%default_args, %in_args);
  foreach my $thiskey (keys(%final_args)) {
    $self->{$thiskey} = $final_args{$thiskey};
  }
  return;
}
-------------- in use_it.pl ---------
use Simple;
my $test = new Simple (a=>1);
# now a==1, b==default.
my $test = new Simple (a=>1, b=>2);
# now a==1, b==2.