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;