Given that the child process is expecting to read something on stdin, you should start it with "open()" rather than with "system()" -- this gives the parent process the ability to continue opertation after the child is opened/started, print data to the child process if/when needed, and also provides the parent with the child's process-id, if you want that, though you could probably just "close" it -- e.g.

my $child_pid = open( CHILD, "| child_program arg1 arg2" ) 
       || die "unable to start child_program\n";

# Feed the child, if that's necessary

select CHILD;
$|++;  # turn off output buffering

print CHILD $/;  # send a blank line

# give the child about 5 seconds to produce something

my $counter = 0;
until ( -s "child_output.file" or ++$counter == 5 ) {
    sleep 1;
}

# now, we could kill $child_pid, or just:

close CHILD;
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Depending on what the child is actually supposed to be doing, there may be some traps here. Definitely read "perldoc -f open" and "perldoc perlipc".