Thanks to everyone who responded. I learned a lot from the suggested code fragments, as well as from Data::Grouper. However, none of these were meeting my needs. Specifically, I wanted to specify more than one fields in both outer and inner loops. Anyway, I finally cracked it, and since I spent two days staring at the screen trying to figure it out, I offer the following with the hope that some other lost soul might find it timesaving and of use.

Assuming I have @res as below (returned from a $sth->fetchall_arrayref({}))

id,maker,make
===============
1,gm,saturn
1,gm,chevy
1,gm,caddy
2,ford,taurus
2,ford,escort
3,vw,bug
3,vw,jetta
3,vw,phaeton

as an array of hashrefs (each row is a hash). And I want a normalized array of hashrefs with nested loops (suitable for H::T). I wrote the following that works (please do suggest any improvements I might be able to make to this) --

my $groupby = 'id';           # field to group by
my @outerfields = ('maker');  # outer loop fields
my @innerfields = ('make');   # inner loop fields
my $cars = &normalize(\@res, $groupby, \@outerfields, \@innerfields);

$template->param(cars => $cars);

# print it all
print "Content-Type: text/html\n\n", $template->output;

sub normalize($aref, $groupby, $outerfields, $innerfields) {
  my ($aref, $groupby, $outerfields, $innerfields) = @_;
  my @res = @$aref;
  my @outerfields = @$outerfields;
  my @innerfields = @$innerfields;
  my $seen = 0;      # a flag to remember seen records
  my @temp;          # a temp array to store outer loop fields
  my %makes;         # a hash to store all the makes keyed by the grou
+pby field

  # loop over the denormalized results
  foreach (@res) {

    # using the groupby field, test if the record has been seen...
    if ($_->{$groupby} != $seen) {

      # create a hash to store the outer loop fields, the car maker
      my %q;
      $q{$groupby} = $_->{$groupby};
      foreach my $of (@outerfields) {
        $q{$of} = $_->{$of};
      }

      # stuff the outer loop fields hash into the temp array
      push(@temp, \%q);

      # create a hash to store the inner loop fields, the car makes
      my %make;
      foreach my $if (@innerfields) {
        $make{$if} = $_->{$if};
      }
      # add the inner loop fields hash to the makes hash keyed by the 
+groupby field
      push(@{$makes{$_->{$groupby}}}, \%make);

      # update the flag
      $seen = $_->{$groupby};

    # since the record has already been seen...
    } else {

      # create a hash to store the inner loop fields, the car makes
      my %make;
      foreach my $if (@innerfields) {
        $make{$if} = $_->{$if};
      }
      # add the inner loop fields hash to the makes hash keyed by the 
+groupby field
      push(@{$makes{$_->{$groupby}}}, \%make);
    }
  }
  
  my @cars;    # an array to hold it all in its final form

  # loop over the temp array created above
  foreach (@temp) {
    # grab the makes for this maker using the groupby field and
    # add it to this row as an array of hashrefs
    $_->{'makes'} = $makes{$_->{$groupby}};

    # add the entire row, a hashref, to the final array
    push(@cars, $_);
  }
  return \@cars;  # return back to the code
}
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Meanwhile, in my html template

<tmpl_loop cars>
 <tmpl_var id>. <tmpl_var maker>
    <tmpl_loop makes>
      <tmpl_var make>
    </tmpl_loop>
</tmpl_loop>
[download]