use Scalar::Util qw(weaken);

{
  my ($sub, $sub1);
  $sub1 = $sub = sub {
    my $num = shift;
    return $num + $sub->($num-1) if $num >0;
    return 0;
  }
  weaken($sub);
  my $num = $sub->(5);
  print "$num\n";
}