my $files = {
   filename1 => {
                   field1 => { start_position => 10,
                               length => 20 
                              }, # values as appropriate
                   field2 => { 
                               start_position => 20
                               length => 5
                              }
                   | 
                   etc. 
                   | 
           },
     filename2 => {
              | and so on...
     }
}