It's not that pretty, but

$template->param(
  some_var => 10,
  (some_var => $hash{some_possibly_undefined_key}) x!! exists($hash{so
+me_possibly_undefined_key}),
);
[download]

will do what you want. Or

$template->param(
  some_var => exists $hash{some_possibly_undefined_key} ? $hash{some_p
+ossibly_undefined_key} : 10;
);
[download]

However, you might also try using Params::Validate

{
   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);
   }
}
[download]

To actually answer your question, some things are hard to protect against and it is best to just try to avoid some constructs. For example you can write

if ($a == 0) 

or you can write

if (0 == $a)

using the second will keep you from the problem of writing if ($a = 0).