$template->param(
  some_var => 10,
  (some_var => $hash{some_possibly_undefined_key}) x!! exists($hash{some_possibly_undefined_key}),
);

##</code><code>##

$template->param(
  some_var => exists $hash{some_possibly_undefined_key} ? $hash{some_possibly_undefined_key} : 10;
);

##</code><code>##

{
   package MyTemplate;

   use base Template;

   use Params::Validate qw (validate);

   sub param
   {
       my $self = shift;
       my %p = validate(@_, {
           some_var => {
               default => 12,
           }
       });
       $self->SUPER::param(%p);
   }
}