I'd second what pc88mxer suggested. — Here's a demo snippet illustrating the idea (Linux version):

#!/usr/bin/perl

my $sid = getppid;  # for the 'ps' process selection only

if (my $pid = fork()) {
    sleep 1; # wait somewhat until tree of children is set up
    # show related processes
    system "ps f -s $sid -o pid,ppid,pgrp,sid,cmd";

    # kill process group after timeout
    sleep 10;
    kill -9, $pid;
    wait;
    print "killed children\n";

    # show remaining processes
    system "ps f -s $sid -o pid,ppid,pgrp,sid,cmd";
    
    # do something else...
    sleep 10;
    
} elsif (defined $pid) {
    # create new process group
    setpgrp;
    # run something (consisting of several processes) that hangs
    system 'bash -c "cat; echo"';
    exit;

} else {
    die "couldn't fork";
}
[download]

Sample output:

$ ./698259.pl
  PID  PPID  PGRP   SID CMD
 2963  2960  2963  2963 bash
 4074  2963  4074  2963  \_ /usr/bin/perl ./698259.pl
 4075  4074  4075  2963      \_ /usr/bin/perl ./698259.pl
 4076  4075  4075  2963      |   \_ bash -c cat; echo
 4077  4076  4075  2963      |       \_ cat
 4078  4074  4074  2963      \_ ps f -s 2963 -o pid,ppid,pgrp,sid,cmd
killed children
  PID  PPID  PGRP   SID CMD
 2963  2960  2963  2963 bash
 4074  2963  4074  2963  \_ /usr/bin/perl ./698259.pl
 4079  4074  4074  2963      \_ ps f -s 2963 -o pid,ppid,pgrp,sid,cmd
[download]

As you can see in the PGRP column, the three child processes in question all belong to the same process group (here 4075).

The idea should in principle even be portable to Windows (never tried it myself though...). As BrowserUk recently noted, that would be Win32::CreateProcess with CREATE_NEW_PROCESS_GROUP.