$path = 'c:/TEST';
opendir DIR, $path or die "Cant open $path, perl says $!\n";
while (my $file = readdir DIR) {
    next if -d "$path/$file" or -l "$path/$file";
    $lc_name = lc $file;
    rename "$path/$file", "$path/$lc_name" or print STDERR "Can't rename $path/$file, perl says $!\n";
}