Cause it complicates things. First, that wouldn't work. You'd have to change the existing waitpid to sleep. You'd end up with:

{
   local $SIG{CHLD} = 'IGNORE';

   ... launch child ...

   if (sleep(20)) {
      kill(KILL => $child_pid);

      # Wait for child to die before restoring $SIG{CHLD}.
      # Unlikely race condition.
      while (kill(0, $child_pid)) {
         sleep(1) or last;
      }
   }
}
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There are side-effects to using $SIG{CHLD} = 'IGNORE'.

• One can't know the exit code of one's children,
• one can't use system, backticks, open '-|' or open '|-', and
• one needs to worry about EINTR.