I think you can use the "-t FILEHANDLE".

From perlfunc:

-t Filehandle is opened to a tty.

If you test STDIN (or maybe STDOUT) with -t and it returns false you should be running from a cron. I think. If you have the perl cookbook i think there is a recipe on this. I don't have it with me. Maybe someone else does.

Formatting fixed by holli