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>
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

Greetings grath,

I worship HTML::Template these days, but one of my early learning curve issues was figuring out nested loops. (This was before I actually RTFMed HTML::Template instead of just skimming, though.) Just remember that everything is a hash, and loops are just a bunch (i.e. array) of hashes. When you have a nested loop, it's just an array of hashes that's the value of a key that's part of the outer loop.

The following code is untested and was pulled from your legacy script and modified. This should give you a place from which to get started.

my $query1 = &db_query("SELECT id, title FROM sections");
my @sections = ();
while (my $rows1 = $query1->fetchrow_hashref()) {
  my $sectionid = $rows1->{id};
  my $query2 = &db_query("SELECT title FROM pages WHERE sectionid = $s
+ectionid");
  my @pages = ();
  while (my $rows2 = $query2->fetchrow_hashref()) {
    push @pages, $rows2;
  }
  push @sections, { pages => \@pages };
}
$template->param(sections => \@sections);
[download]

I hope this helps. Try using Dump::Dumper to see the created data structure before sending it to HTML::Template. Actually seeing the data structure has helped me on many occations. Also, your legacy script doesn't use strict or use warnings. I recommend you put those in there.

UPDATE: Use jeffa's code from his post. It's far better than mine because he rewrote the database interface using more standard DBI syntax. Use my code above only as a learning tool to map to your original script.

my @outer;
while (my $rows1 = $query1->fetchrow_hashref()) {
   my $query2 = &db_query("SELECT title FROM pages WHERE sectionid = $
+sectionid");
   $inner=[]; # you need a NEW array for each loop, not just  an empty
+ array

   while (my $rows2 = $query2->fetchrow_hashref()) {
      push(@$inner,{TITLE=>$rows2->{title});
   }

   push(@outer,{
                 TITLE=>$rows1->{title},
                 PAGES=>$inner;
               });
}
$template->param(sections=>\@outer);
[download]