I wouldn't use the 'ctime' field (nor '-C' for that matter) since it's implementation varies depending on platform, and doesn't necessarily indicate age of a file's data -- just the file's metadata. You'll probably want to to use mtime (write time) or atime (access: most recent read or write) instead. See "perldoc perlport" and look for ctime.
in reply to Finding oldest file in directory
I also wouldn't consider optimizing this much since any reasonable OS will cache your stats for you (and you only have a few files). These stats may even stay cached if you are reading them once a minute.