First off please use code tags so your code is readable - I have done this for you this time around.

my $pid = fork; 
if ($pid > 0){ 

     eval{ 
         local $SIG{ALRM} = sub {kill 9, -$PID; die "TIMEOUT!"}; 
         alarm $num_secs_to_timeout; 
         waitpid($pid, 0); 
         alarm 0; 
     }; 

} elsif($pid == 0){ 
       setpgrp(0,0); 
       exec('echo blahblah | program_of_interest'); 
       exit(0); 
}
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Documentation for fork will show you that the fork call returns the pid of the forked process to the parent and 0 to the child. This is used above to run the parent through the if and the child through the else above.

The following should help you understand the rest:

1. setpgrp
2. exec
3. Piped redirection of input and output
4. How to use Alarm to timeout

This code essentially creates a process which executes a non-perl program on a timeout.