The error probably has to do with the use of forward slashes in the filename, which appear to be interpreted as switches by rmdir.

I strongly recommend you use remove_tree from File::Path, as several others have suggested. It has been a core module for over 20 years and should therefore not only meet your requirement of being "plain Perl", but unlike rmdir it is also platform-independent!

Also, note that you probably don't want to use a "delete directory" function to delete individual files. See unlink for files.