sub new_callback_accessor {
   my $start    =  $_[0];
   my $end      =  $_[1];
   my $data_ref = \$_[2];  # Avoid making a copy.

   return sub {
      my ($callback, $blk_size) = @_;
      local *_;
      $blk_size = 100 unless defined $blk_size;
      $blk_size = $end-$start unless $blk_size;
      my $ofs = $start;
      my $len = $end-$start;
      while ($len) {
         $blk_size = $len if $blk_size > $len;
         $_ = substr($$data_ref, $ofs, $blk_size);
         $callback->();
         $ofs += $len;
         $len -= $blk_size;
      }
   };
}

{
   my $data = ...;

   my $start1 = ...;  # Calculate start from map.
   my $end1   = ...;  # Calculate end   from map.

   my $rec1 = new_callback_accessor($start1, $end1, $data);
   $rec1->(sub { print });
   print("\n");
}