my $dbh = DBI->connect(
   qw(DBI:mysql:database:host user pass), #substitute your values here
   {RaiseError => 1 },
);

my $sections = $dbh->selectall_arrayref('
   SELECT id, title FROM sections
',{Slice => {}});

for my $section (@$sections) {

   my $pages = $dbh->selectall_arrayref('
      SELECT SELECT title FROM pages WHERE sectionid = ?
   ',{Slice => {}}, $section->{id});
 
   $section->{pages} = $pages;
}

my $tmpl = HTML::Template->new(filehandle => \*DATA);
$tmpl->param(sections => $sections);

print $tmpl->output;

__DATA__
<ul>
   <tmpl_loop sections>
      <li>Section: <tmpl_var title>
      <ul>
         <tmpl_loop pages>
            <li>Page: <tmpl_var title></li>
         </tmpl_loop>
      </ul>
   </tmpl_loop>
</ul>