When a file was written to a directory is not the same thing as when it was last modified. I'd think you'd want the file creation time, and the Perl stat command doesn't seem to have that:

  8 atime    last access time since the epoch
  9 mtime    last modify time since the epoch
 10 ctime    inode change time (NOT creation time!) since the epoch
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And neither does the Unix stat() command:

struct stat {
    ,,,
    time_t        st_atime;    /* time of last access */
    time_t        st_mtime;    /* time of last modification */
    time_t        st_ctime;    /* time of last change */
};
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So in answer to your question, you're pretty much out of luck unless (a) you have control of the writing and can add a few lines to a script to store the write times or (b) you know that files will only be written, not modified, in which case modification time will do fine.

EDIT: Incidently, your answer was completely useless, BUU. If it's so easy to find the info, you might as well copy and paste it into your post instead of flaming the poster. --