#!/usr/bin/perl
use strict;
use warnings;

my %tree = (
    FOO   => [qw/BAR BLAH ASDF/],
    BAR   => [qw/LA/],
    BLAH  => [qw/XYZ/],
    ASDF  => [qw/OOOOO/],
    LA    => [],
    XYZ   => [qw/LA/],
    OOOOO => []
);
{
  my %seen;
  my %onstack;
  my @list;

  sub how_to_uninstall {
      my $target = shift;
### COMMENT OUT THE FOLLOWING LINE ###
#      (@list, %seen, %onstack) = ();

      print "HOWTO ($target) START\n";
      _traverse($target);

### NEW STUFF ###
for my $token (keys(%tree)) {
    for (@{$tree{$token}}) {
        if (defined($seen{$_}) != 1) {
            how_to_uninstall($token);
        }
    }
}
### END NEW STUFF ###

      print "HOWTO ($target) FINISH\n";
      return @list;
  } 

  sub _traverse {

      my $x = shift;
      print "  TRAVERSE ($x) START\n";

      $seen{$x} = $onstack{$x} = 1;
      for my $y (@{$tree{$x}}) {
          die "cyclic!" if $onstack{$y};    # back edge
          print "    Found $y\n";
          _traverse($y) unless $seen{$y};
      }
      push @list, $x;
      $onstack{$x} = 0;
      print "  TRAVERSE ($x) FINISH\n";
  }
}

my @order = how_to_uninstall('BLAH');
print "\nANSWER = @order\n";

##</code><code>##

{C} > 808487.pl
HOWTO (BLAH) START
  TRAVERSE (BLAH) START
    Found XYZ
  TRAVERSE (XYZ) START
    Found LA
  TRAVERSE (LA) START
  TRAVERSE (LA) FINISH
  TRAVERSE (XYZ) FINISH
  TRAVERSE (BLAH) FINISH
HOWTO (ASDF) START
  TRAVERSE (ASDF) START
    Found OOOOO
  TRAVERSE (OOOOO) START
  TRAVERSE (OOOOO) FINISH
  TRAVERSE (ASDF) FINISH
HOWTO (FOO) START
  TRAVERSE (FOO) START
    Found BAR
  TRAVERSE (BAR) START
    Found LA
  TRAVERSE (BAR) FINISH
    Found BLAH
    Found ASDF
  TRAVERSE (FOO) FINISH
HOWTO (FOO) FINISH
HOWTO (ASDF) FINISH
HOWTO (BLAH) FINISH

ANSWER = LA XYZ BLAH OOOOO ASDF BAR FOO