use Data::Dumper;

   my $foo = "a:b:c:d:a";
   my $bar = "a:b:c:d:z";
   my $hoh = {};

   sub createHash {
      my ($hoh,$str_orig,$str_rest,$lastkey,$parent) = @_;

      $str_rest = $str_rest || $str_orig || "";
      $_        = $str_rest;

      if (/^(.*?):(.*)$/)
      { 
         $parent    = $hoh;
         $hoh->{$1} = $hoh->{$1} || {};
         createHash($hoh->{$1},$str_orig,$2,$1,$parent);
      } 
      elsif (defined($lastkey)) 
      {
         delete($parent->{$lastkey}) if ref $parent->{$lastkey} ne "ARRAY";
         push (@{$parent->{$lastkey}} , [$str_rest,$str_orig]);
      }
      return $hoh;
   }
   $hoh = createHash($hoh,$foo);
   $hoh = createHash($hoh,$bar);

   print Dumper($hoh);