One minor problem with the above code.. The opendir command is done relative to the current directory. If you were to opendir('dir1'), you would be looking in pwd/dir1. If you then found a directory inside of dir1 named dir2, an opendir('dir2') would look for pwd/dir2, which is not the desired result. To get the recursion right, the recurse_dir would have to look something like

sub recurse_dir {
   my $dir = shift();

   opendir D, $dir;
   while (readdir D) {
      process_file ($dir/$_) if -f $dir/$_;
      recurse_dir  ($dir/$_) if -d _;
   }
   closedir D;
}
[download]

Either that, or the recurse_dir function could chdir in to each directory and chdir .. after the while loop.