An often overlooked concept are process group IDs.  Just create a new process group before you fork. Then, independently of how many children are being forked, and how deep their fork/exec level gets, you can address all of them in one go.  A short demo:

#!/usr/bin/perl

my $sid = getppid;  # for the 'ps' 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 5;
    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 5;
    
} 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";
}

__END__

Sample output:

$ ./726031.pl
  PID  PPID  PGRP   SID CMD
29171 29169 29171 29171 bash -rcfile .bashrc
  508 29171   508 29171  \_ /usr/bin/perl ./726031.pl
  509   508   509 29171      \_ /usr/bin/perl ./726031.pl
  510   509   509 29171      |   \_ bash -c cat; echo
  511   510   509 29171      |       \_ cat
  512   508   508 29171      \_ ps f -s 29171 -o pid,ppid,pgrp,sid,cmd
killed children
  PID  PPID  PGRP   SID CMD
29171 29169 29171 29171 bash -rcfile .bashrc
  508 29171   508 29171  \_ /usr/bin/perl ./726031.pl
  513   508   508 29171      \_ ps f -s 29171 -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 509). The single kill addressed to this group gets rid of all three processes.

The only issue you might run into is when some process further down the line itself creates another new process group. But as the concept is largely unknown/unused, the chances you'll encounter that problem in practice are rather low :)  At least, it's worth a try.

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