my $stmt = 'SELECT title, id FROM stages_stage ORDER BY id';
my $stages = $dbh->selectall_arrayref($stmt, {Slice => {}});
	   
$stmt = 'SELECT title, stage_id FROM stages_topic ORDER BY stage_id';
my $topics = $dbh->selectall_arrayref($stmt, {Slice => {}});
	   
for my $i ( 0 .. $#$stages ) {
   $list->[$i]{'stage_title'} = $stages->[$i]{'title'};
   $ictr = 0;

   while ( $topics->[$tctr]{'stage_id'} == $stages->[$i]{'id'} ) {
      $list->[$i]{'topics'}->[$ictr]{'topic_title'} = $topics->[$tctr]{'title'};
      $tctr++;
      $ictr++;
   }
}

Dumper:
$VAR1 = [
          {
            'stage_title' => 'Visualize Your Life',
            'topics' => [
                          {
                            'topic_title' => 'Your Mind'
                          },
                          {
                            'topic_title' => 'Your Identity'
                          },
                          {
                            'topic_title' => 'Your Security'
                          }
                        ]
          },
          {
            'stage_title' => 'Choose Your Buyer',
            'topics' => [
                          {
                            'topic_title' => 'Yourself'
                          },
                          {
                            'topic_title' => 'Employee(s)'
                          },
                          {
                            'topic_title' => 'Co-Owner'
                          },
                          {
                            'topic_title' => 'Family'
                          },
                          {
                            'topic_title' => 'Third-Party'
                          }
                        ]
          },
        ];