The first thing that sticks out is your children are not exiting properly. Once they finish their job, they go back into the foreach loop and fork off new processes as if they were the parent. This causes numerous pings for each host. First thing to do then, is to add an exit call right after the child code...

for ( [snip] ) {
  $pid[$_] = fork;
  if ($pid[$_]) {
      [snip]
  } else {
      # ping some hosts
      exit;   # <-- important that your children exit...
  }
}
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-Blake