$tmpl->param(
  foos     => \@foo,
  num_foos => scalar @foo,
  bars     => \@bar,
  num_bars => scalar @bar,
  ...
);

##</code><code>##

use HTML::Template;

sub tmpl_num_filter {
    s[<TMPL_NUM (\w+)>]
     [<TMPL_IF $1><TMPL_LOOP $1><TMPL_IF __LAST__><TMPL_VAR __COUNTER__></TMPL_IF></TMPL_LOOP><TMPL_ELSE>0</TMPL_IF>]g
        for ${+shift};
}

my $tmpl = HTML::Template->new(
               scalarref         => \do { local $/; <DATA> },
               filter            => \&tmpl_num_filter,
               die_on_bad_params => 0,
               loop_context_vars => 1
           );

$tmpl->param(hobbits => [
    { name => "Frodo" },
    { name => "Samwise" },
    { name => "Meriadoc" }
]);
print $tmpl->output;

__DATA__
These are my <TMPL_NUM hobbits> favorite hobbits:
<ul><TMPL_LOOP hobbits>
  <li><TMPL_VAR name>
</TMPL_LOOP></ul>