use strict;
use warnings;

use Algorithm::Loops qw( NestedLoops );

my $tree = {
  A => '',
  B => 'A',
  C => 'A',
  D => 'C',
  E => 'C',
  F => 'B',
  G => 'D',
  H => 'D',
  I => 'D',
  J => 'G',
  K => 'B',
  L => 'K',
  M => 'L',
  N => 'M',
  O => 'N',
};

sub get_descendants {
  my ($tree, $node) = @_;
  my @children;
  for my $k (keys %$tree) {
      push @children, $k if $tree->{$k} eq $node;
  }
  return \@children;
}

my @paths = NestedLoops(
    [   sub { get_descendants($tree,'') },
       (sub { get_descendants($tree,$_) } ) x (scalar(keys %$tree) - 2)
    ],
    {  OnlyWhen => 1
    },
    sub { \@_ }
);

print join(', ',@$_) . "\n" for (@paths);

__END__
A
A, B
A, B, F
A, B, K
A, B, K, L
A, B, K, L, M
A, B, K, L, M, N
A, B, K, L, M, N, O
A, C
A, C, E
A, C, D
A, C, D, H
A, C, D, I
A, C, D, G
A, C, D, G, J