btw,

sub scope_test{
  my @data_in = <DATA>;
  my @data_out;
  for (0..$#data_in){
    call_back(\@data_out, $data_in[$_]);
  }
  print "$_" for @data_out;
}
[download]

can be simplified to

sub scope_test{
  my @data_out;
  while (<DATA>) {
    call_back(\@data_out, $_);
  }
  print "$_" for @data_out;
}
[download]

or even

sub scope_test{
  my @data_out;
  call_back(\@data_out, $_) while (<DATA>);
  print "$_" for @data_out;
}
[download]