package BaseType;

...

BEGIN {
  my $parameter_config = undef;

  # accessor - turns the ParamConfig object into a closure
  sub parameter_config {
    $parameter_config = $_[0] if @_;
    return $parameter_config;
  }
}

sub new {
  my $class = shift;      # the usual prolog
  my $type = ref $class || $class;

  my $self = bless {}, $type;
  return $self;
}

...