It may have if you got @files by something like:

opendir($dh, "$working_directory"
@files = grep { /\.csv$/i } readdir($dh)
[download]

and didn't close the directory. I know rmdir doesn't work on Windows if the directory being deleted is open somewhere; Windows may prevent writing the directory if it's still "open" by opendir.

On the other hand...if you got the names by some other means, I'll admit being stumped.