Hi, I want to know how we can use callback functions within perl for a scenario as follows. I have a parent process with N number of child's.

You don't want callbacks, you want signals.

The parent sleeps for some time and returns back to check whether any child is alive. If it is then it says "Processing" and goes back to sleep.

After you've forked your children, handle the SIGCHLD signal appropriately to be notified when a child process has exited.

If all the children are completed, the parent exits with saying a status of "Completed".

As you handle the signal from each child process, remove its process id from a hash you filled when you forked the processes.

If you need to pass data to/from the processes, you can use one of many mechanisms described in perlipc.

Or you can do all of that with a simple OO interface: Parallel::ForkManager. :-)

I want to know how callback functions can be used for this kind of scenario

You can only really use callbacks in the same process and thread. You can use a construct approximating callbacks under threads.

-David