I don't know why, but I find -M and friends confusing, because they work the 'wrong' way for my brane, by making older files have higher values, whereas stat and friends use epoch time and newer files have higher values. So I'd use stat like below. There's a little more code, but it's easier for me to read:

unlink grep { (stat($_))[9] < time - ($days * 86400) } @files;
[download]

where 86400 is the number of seconds in a day, so we unlink all files from the @files array where their modification time earlier than $days days old.