$data{$sub_alias}{$topic_sub}{$id}{ 'URL' } = 'my_url';
[download]

creates a %data-hash (not a $data reference to a hash - I hope you know what I mean).

So do get to get to the $id-values you do need to get rid of the "->":

foreach $tmp_sa (@foo)  {

   foreach $t (@bar)..... {

      foreach ( keys %{$data{$tmp_sa}{$t}} ) {
         print "$data{$tmp_sa}{$t}{$_}{'URL'}\n";
      } 

   }
[download]