Just save the pids as you generate them:

my @pids;
for loop {
  $pid = fork();
  if ($pid) { # if parent
    push(@pids, $pid);
  } else { # if child
    ...
    exit;
  }
}
# only parent reaches this point

kill 9, @pids; # perform mass filicide
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or... have waitpid() tell you what they are:

use POSIX qw(:syswait_h);

while ((my $pid = waitpid(-1, WNOHANG)) != -1) {
  kill 9, $pid;
}
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