<TMPL_LOOP NAME="MAINLOOP">
  <b><TMPL_VAR NAME="MAIN"></b><br>
  <TMPL_LOOP NAME="SUBLOOP">
    <TMPL_VAR NAME="SUBS"><br>
  </TMPL_LOOP>
</TMPL_LOOP>

my @mainloopdata = ();
foreach my $main (@mains)
{
   my %mainhash = ();
   $mainhash{MAIN} = $main->{MAIN};
   my @subsloopdata = ();
   foreach my subs (@{$main->{SUBS}})
   {
      my %subhash = ();
      $subhash{SUBS} = $subs;
      push @subsloopdata, \%subhash;
   }
   $mainhash{SUBLOOP} = \@subsloopdata;
   push @mainloopdata , \%mainhash;
}
$tpl->param(MAINLOOP => \@mainloopdata);