You want to know the child's PID in the child? $$.
You want to know the parent's PID in the child? Just store $$ into a variable (say $ppid) before the loop.
You want to know the child's PID in the parent? Use the snippet below.

for (...) {
   my $pid = $pm->start;
   if ($pid) {
      # We're in the parent.
      ...[ do something with $pid ]...
      next;
   }

   # We're in the child.
   ...
}
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Update: Added third question and answer.