Nothing to do with your problem, but you do not need to double-up the backslashes and even don't need to use back-slashes in the filepath.
my $rootdir = 'C:/Users/Nathaniel/Corpus/';
works equally well and is less typing.
A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James
My blog: Imperial Deltronics