sub chfile
  {
  .......
  my $datafile = "myfile.txt";
  my $filename = PerlApp::extract_bound_file($datafile);
  die "$datafile not bound to application\n" unless   
  defined $filename;
  my $fh="LEX";
  open($fh,'$filename') or die "Can't open $datafile
  $filename)\n";
  
  while(<LEX>){
  ......

  }
 }