G'day Karl,

My main issues with the way you're currently doing this is that you're reading the entire process table for every ancestry level you want and, for each of those levels, you're repeating almost identical code.

Here's a technique that only reads the process table once and allows access to the process data in a variety of ways (I've shown a few examples). I've used your data (and some extra for testing) in a dummy AoH reference ($table) rather than attempting to recreate a Proc::ProcessTable object: you'll need to make changes (e.g. '$_->{pid}' to '$_->pid') for a real solution. Also note that by changing the value of $ancestry, you can get whatever number of ancestry levels you want; although, you may want to rethink my constant names if you start using 'GREATGREAT...GREATGRANDPARENT'. :-)

#!/usr/bin/env perl

use strict;
use warnings;

use constant {
    CHILD            => 0,
    PARENT           => 1,
    GRANDPARENT      => 2,
    GREATGRANDPARENT => 3,
};

my $table = [
    { pid => 0,     ppid => 0,     cmd => 'root' },
    { pid => 603,   ppid => 0,     cmd => 'cron_parent' },
    { pid => 18421, ppid => 603,   cmd => 'cron_child' },
    { pid => 18423, ppid => 18421, cmd => 'sh_mon' },
    { pid => 18425, ppid => 18423, cmd => 'sh_kit' },
    { pid => 18444, ppid => 18425, cmd => 'java' },
    { pid => 28421, ppid => 603,   cmd => 'cron_child' },
    { pid => 28423, ppid => 18421, cmd => 'sh_mon' },
    { pid => 28425, ppid => 18423, cmd => 'sh_kit' },
    { pid => 28444, ppid => 18425, cmd => 'java' },
    { pid => 28445, ppid => 18425, cmd => 'java' },
    { pid => 99999, ppid => 18421, cmd => 'java' },
    { pid => 2,     ppid => 1,     cmd => 'unwanted' },
];

my $ancestry = 3;
my %tree; 

{
    my %all;

    for (@$table) {
        $all{$_->{pid}} = $_;
        $tree{$_->{pid}} = [ $_ ] if $_->{cmd} =~ /java/;
    }

    for my $child (keys %tree) {
        my $ppid = $all{$child}{ppid};

        for (1 .. $ancestry) {
            push @{$tree{$child}}, {%{$all{$ppid}}};
            $ppid = $all{$ppid}{ppid};
        }
    }
}

use Data::Dump;
print "*** All parents ***\n";
dd [ map { $tree{$_}[PARENT] } sort keys %tree ];

print "*** 18444 grandparent ***\n";
dd $tree{18444}[GRANDPARENT];

print "*** 18444 greatgrandparent ***\n";
dd $tree{18444}[GREATGRANDPARENT];

print "*** All ancestries ***\n";
dd \%tree;
[download]

Output:

*** All parents ***
[
  { cmd => "sh_kit", pid => 18425, ppid => 18423 },
  { cmd => "sh_kit", pid => 18425, ppid => 18423 },
  { cmd => "sh_kit", pid => 18425, ppid => 18423 },
  { cmd => "cron_child", pid => 18421, ppid => 603 },
]
*** 18444 grandparent ***
{ cmd => "sh_mon", pid => 18423, ppid => 18421 }
*** 18444 greatgrandparent ***
{ cmd => "cron_child", pid => 18421, ppid => 603 }
*** All ancestries ***
{
  18444 => [
             { cmd => "java", pid => 18444, ppid => 18425 },
             { cmd => "sh_kit", pid => 18425, ppid => 18423 },
             { cmd => "sh_mon", pid => 18423, ppid => 18421 },
             { cmd => "cron_child", pid => 18421, ppid => 603 },
           ],
  28444 => [
             { cmd => "java", pid => 28444, ppid => 18425 },
             { cmd => "sh_kit", pid => 18425, ppid => 18423 },
             { cmd => "sh_mon", pid => 18423, ppid => 18421 },
             { cmd => "cron_child", pid => 18421, ppid => 603 },
           ],
  28445 => [
             { cmd => "java", pid => 28445, ppid => 18425 },
             { cmd => "sh_kit", pid => 18425, ppid => 18423 },
             { cmd => "sh_mon", pid => 18423, ppid => 18421 },
             { cmd => "cron_child", pid => 18421, ppid => 603 },
           ],
  99999 => [
             { cmd => "java", pid => 99999, ppid => 18421 },
             { cmd => "cron_child", pid => 18421, ppid => 603 },
             { cmd => "cron_parent", pid => 603, ppid => 0 },
             { cmd => "root", pid => 0, ppid => 0 },
           ],
}
[download]

[While I can see the Proc::ProcessTable documentation uses indirect object notation, please be aware this syntax is strongly discouraged (explained in perlobj: Invoking Class Methods).]