So you could:

sub name {
  die 'ref arg required in void context' if ! defined wantarray and ! 
+ref $_[0];
  my ($arg) = @_;
  if (ref $arg) {
    $$arg = 'x';
    return;
  }

  return 'x';
}
[download]

as a check that you've got a ref param in void context.