#!/usr/bin/perl

use strict;
use Data::Dumper;
my $form = "%30s  %s\n";

my %upgrades;
my %lvl;
my %PofC = (
    1   => 2
,   3   => 2
,   4   => 2
,   2   => 5
,   6   => 5
,   7   => 4
,   5   => undef
,   8   => 4
);
print Dumper \%PofC;

foreach my $child (keys %PofC) {
    $lvl{$child} = 0;
}

my $leftovers = 1;
my $level = 0;
while ($leftovers > 0) {
    %upgrades = ();
    $leftovers = 0;
    foreach my $child (keys %PofC) {
        if (exists $lvl{$PofC{$child}}
        &&  $lvl{$PofC{$child}} == $level)
        {
            $upgrades{$child} = 1;
            ++$leftovers;
        }
    }
    foreach (keys %upgrades) {
        ++$lvl{$_};
    }
    ++$level;
}

print Dumper \%lvl;

printf $form, 'child', 'parent';
foreach my $c ( sort { $lvl{$a} <=> $lvl{$b} } keys %lvl ) {
    printf $form, $c, $PofC{$c};
}


exit;

__OUTPUT__
# %PofC
$VAR1 = {
          1 => 2,
          2 => 5,
          3 => 2,
          4 => 2,
          5 => undef,
          6 => 5,
          7 => 4,
          8 => 4
        };
# %lvl
$VAR1 = {
          1 => 2,
          2 => 1,
          3 => 2,
          4 => 2,
          5 => '0',
          6 => 1,
          7 => 3,
          8 => 3
        };
                         child  parent
                             5  
                             2  5
                             6  5
                             4  2
                             3  2
                             1  2
                             7  4
                             8  4