#!/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";