The code you posted does not even have 21 lines, so it would be better to post the code you're actually using, or to use the code you actually post. The code as posted does not even compile - $old_filename is never declared. On the other hand, $file is never used.

Most likely though, you will find some help if you change your error message to be more informative:

symlink("$path/$old_filename$ext", "$path/$new_filename" )
    || die "can't create link from $path/$old_filename$ext to $path/$n
+ew_filename : $!\n";
}
[download]

... except that you don't even tell us whether your program works or not.