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

   return sub {
      return substr($$data_ref, $start, $end-$start);
   };
}

{
   my $data = ...;

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

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