Well first off, SIGKILL (9) is untrappable and unignorable. You could choose to send SIGTERM (15) which has always been the *nicer* way to terminate a process. If you choose to do that, then you would need to setup a signal handler in your parent process that would then send a signal to the process group. (This, of course, all depends on how your platform handles signals and processes!):

#!/usr/local/bin/perl
# parent code
use strict;
use warnings;

my $pid;

$SIG{TERM} = \&catch_sig;

if( $pid = fork ) {
  # parent
  while( 1 ) {
    print "parent: $$, ", getpgrp, "\n";
    sleep 2;
  }
} else {
  exec ('./child.pl') or die "couldn't exec child.pl: $!";
}

sub catch_sig {
  kill -15, getpgrp;
  exit;
}

#!/usr/local/bin/perl

#child code --- nothing special here
use strict;
use warnings;

while( 1 ) {
  print "child: $$, ", getpgrp, "\n";
  sleep( 3 );
}
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Now if you send kill -15 parent_process_id from the shell, the signal is trapped and then propagated to the group.