class TheHulk;

sub new {
  bless $self, $class;
  $self->go_normal;
  $self;
}

sub go_normal {
  bless $self, TheHulk::_normal;
}

sub go_green {
  bless $self, TheHulk::_green;
}

sub relax {}

sub make_angry {}

package TheHulk::_normal;

sub run {
  ...
}

sub make_angry {
  $self->go_green;
}

package TheHulk::_green;

sub run {
  ...  
}

sub relax {
  $self->go_normal;
}