foreach my $file (@files) {
  my  $outfile_fname = $file.".html";     # output file name
  open ( my $out_handle, '>', "result_dir/$outfile_fname" )
        or croak "$0 : failed this file $outfile_fname : $!\n";

  my $result  = process($file);

  # You should check the return value on any file I/O operation 
  print $out_handle $result or croak "Failed to print to $outfile_fname";
 
  close ( $out_handle ) or croak "Failed to close $outfile_fname";
}