my $function = sub {
    # ...do stuff...
};

# Call the sub
$function->('param');

##</code><code>##

sub function {
    # ...do stuff...
}

# Call the sub
function('param);

##</code><code>##

my $vars = {
    'date'    => $date,
    'format'  => sub { return ucfirst lc $_[0]; },
};
$template->process('example.tt', $vars);