Why is SIGCHLD affecting sleep?

Select or download this

$SIG{CHLD} = sub { };

...
sub foo {
  print '*' for(1..5);
}

Select or download this

**************************************---Slept for 2 seconds
**************************************---Slept for 2 seconds
**************************************---Slept for 2 seconds
**************************************---Slept for 2 seconds
**************************************---Slept for 2 seconds

Select or download this

**************************************---Slept for 2 seconds
**************************************---Did not sleep at all
...
*******************************************---Slept for 2 seconds
**********************************---Did not sleep at all
*******************************************---Did not sleep at all

Select Code to Download