|  in your while loop invoke:
        process_file($tmp_dir,$file);
| and then after your loop

sub process_file {
   my($path,$node)=@_;

   my $fname = $path . '\' . $node;
   open FIN,"<$fname" or die "$fname:$!";
   while (my $line=<FIN>){
       chomp $line;
       |  do something here. 
   }
}