There must be something more here eff_i_g for it to be important to distinguish between void and scalar context.

Your sample code doesn't distinguish between void and scalar context of course. It simply provides a (somewhat hacky) way of signaling to the sub that you don't want a value returned. You could do something similar in a rather more up front way by:

do_something_to(\$this_var, 'No return required');

sub do_something_to {
if (! $#_)
  {
  $$_[0] = 'x';
  return;
  }

return 'x';
}
[download]

but that's pretty ugle really :).