You could open the other file ("--"?) from perl, not direct it to a file and use open to read from it, or pipe your ksh output through tee or something. Unless you're planing to read the output of your fork using open (instead of system), this might not even be a perl question precisely.

An update based on the comments below... Perhaps if you included more info about your actual system call (for example what you're calling or the precise syntax you're using) we could help more. Presently I doubt the use of "--" does what you think it does.