#!/usr/bin/perl -w
use strict;
use HTML::Template;

# open the html template
my $doc = <<ENDE
<TMPL_LOOP name="outer_loop">
    <td>
    <TMPL_VAR name="schluessel">
      <TMPL_LOOP name="inner_loop">
        <br /><TMPL_VAR name="daten">
      </TMPL_LOOP>
    </td>
</TMPL_LOOP>
ENDE
    ;   
my $template = HTML::Template->new(scalarref => \$doc);

# Beispiedaten
my %thehash = ( 
  Erster_Schluessel => [ 'a1', 'a2' ],
  Zweiter_Schluessel => [ 'b1', 'b2', 'b3' ]
);

my (@outer_loop, @inner_loop, $schluessel, $daten);

foreach $schluessel (keys %thehash) {
  my %outer_loop_row;
  foreach my $daten (@{$thehash{$schluessel}}) {
    my %inner_loop_row;
    $inner_loop_row{"daten"} = $daten;
    push (@inner_loop, \%inner_loop_row);
}
%outer_loop_row =(
  schluessel => $schluessel,
  inner_loop => \@inner_loop
);
push (@outer_loop, \%outer_loop_row);
}

$template->param(outer_loop => \@outer_loop);

print $template->output;