call_back(\@data_out, $data_in[$_]);

sub call_back{
  my ($data_out, $element) = @_;
  push @$data_out, $element;
}

##</code><code>##

call_back(\@data_out, $data_in[$_]);

sub call_back{
  our @data_out; local *data_out = $_[0];
  my  $element                   = $_[1];

  push @data_out, $element;
}