There is an option to allow exactly what you're looking for. In fact it's one I ALWAYS have set. From the docs:
die_on_bad_params - if set to 0 the module will let
you call $template->param(param_name => 'value') even
if 'param_name' doesn't exist in the template body.
Defaults to 1.
Change your constructor to
$tmpl = HTML::Template->new('filename' => $TEMPLATE,
'die_on_bad_params' => 0
And you'll be good to go.
perl -l -e "eval pack('h*','072796e6470272f2c5f2c5166756279636b672');"