[% # THis is in templatelib.tt %]
[% MACRO date_template(parm1, parm2, parm3) BLOCK %]
  [% SWITCH parm1 %]
    [% CASE "value 1" %]
      display this way
    [% CASE "value 2" %]
      display other way
    [% CASE DEFAULT %]
      display default way
  [% END %]
[% END %]

##</code><code>##

[% # this is foo.tt %]
[% PROCESS templatelib.tt %]
[% date_template(parm_from_script, "foo", "bar") %]

##</code><code>##

my $template = Template->new();

my $t_vars = {
   parm_from_script => "value 1",
   other_stuff => "mmmm donuts",
};

$template->process("foo.tt", $t_vars) ||
   die $template->error();