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{
  my %cache;
...
      defined $cache{ $arg } ? $cache{ $arg }
                             : $cache{ $arg } = grunt($arg);
}

sub heavy_lifting {
  my $self = shift;
...
  return defined $self->{ _heavy } ? $self->{ _heavy }
                                   : $self->{ _heavy } = $self->grunt(
+ $arg );
}