In short, no, CGI won't do that for you, because that's not really its job. You can do it yourself pretty easily though. I'm going to simplify your example a little by using a dot instead of curlies.

<input name="form_item.foo"    value="one">
<input name="form_item.bar"    value="two">
<input name="form_item.fiddle" value="three">
[download]

Now you can "split" on the dot to nest the hashes. The code below works recursively, so you can have multiple dots in a field name and they will be broken into several levels of hashes.

sub nest_hash
{
  my ($delim, $hash) = @_;
  my $new;

  while (my ($key, $value) = each %$hash) {
    unless ($key =~ /\Q$delim\E/s) {
      $new->{$key} = $value;
      next;
    }

    my ($before, $after) = ($`, $');
    my $temp = nest_hash($delim, { $after => $value });
    while (my ($k, $v) = each %$temp) {
      $new->{$before}{$k} = $v;
    }
  }
  return $new;
}

my $params = $q->param();
my $nested = nest_hash('.', $params);
my $hashref = $nested->{form_item};

print $nested->{form_item}{foo};     # one
print $hashref->{fiddle};            # three
[download]

Code is only partially tested.