$hash{"0"}=["1","2","15"];
  $hash{"1"}=["3","4"];
  $hash{"2"}=["5","6"];
  $hash{"3"}=["7","8"];


  sub expand
  {
        my ($key) = @_;
        if (exists $hash{$key})
        {
              return sort { $a <=> $b } $key, map { expand($_) } @{$hash{$key}}
        }
        else
        {
              return $key
        }
  }

  my @expanded = map {[ expand $_ ]} @{$hash{0}};

  use Data::Dumper;
  print Dumper(\@expanded);

##</code><code>##

  sub expand
  {
        my ($key) = @_;
        return sort { $a <=> $b } $key, map { expand($_) } @{$hash{$key}}
  }

##</code><code>##

  sub e{@_,map{e($_)}@{@hash{@_}}}
  print Dumper[map{[sort{$a<=>$b}e$_]}@{$hash{0}}]