Cron runs cronjobs at periodic intervals (that's it's job); there's not really any connection between filesystem events and it. You certainly could have a program (that cron runs periodically) which looks at the contents of a directory (using readdir and friends) and compares that to a list kept in a file, but the cron-ness has nothing to do with it really (comparing apples and badgers).

The other alternative would be to use something like Sys::Gamin or the like in a persistent program which runs constantly until the OS notifies it that a directory has changed, but again that's got nothing to do with cron.