The problem is that the readline blocks until it has read a line (irrespective of whether you use it in list or scalar context). So despite your non-blocking waitpid(-1,WUNTRACED) you won't start a new child before the old one has written a line to the pipe. In other words, as you have it, your pings will still run sequentially (just as they would without the additional fork/wait wrapped around the backticks call).

If you want to run several child processes in parallel, you'd have to have independent pipes, and (for example) read from them in a select loop when they're ready (the other form of select, that is, not the one you're already using in your code).