How to pass optional parameters to a Perl function?

Question

I want to pass several parameters, one of which is optional, to a function. The only way to do it that I know is using a list (@) as a parameter. Thus, it contents nothing or 1 element (will never be undef), so that I can use the following code:

sub someFunction($$@) {
    my ( $oblig_param1, $oblig_param2, $option_param ) = @_;
    ...
} 

This code works, but I feel that maybe it's not the best workaround.
Are there any other ways to do it?
Thank you.

Answer 1

Prototypes (the ($$@) part of your sub declaration) are optional themselves. They have a very specific use, and if you don't know what it is, it is better to not use it. From perlsub:

...the intent of this feature is primarily to let you define subroutines that work like built-in functions

Just remove the prototype from your sub declaration, and you can use whatever arguments you like.

sub someFunction {
    my ( $oblig_param1, $oblig_param2, $option_param ) = @_;
    if (defined $option_param) {
        # do optional things
    }
    $option_param //= "default optional value";
    ....
} 

Answer 2

You can use a semicolon in the prototype to indicate the end of the required parameters:

sub someFunction($$;$) {
  my ( $oblig_param1, $oblig_param2, $option_param ) = @_;
  ...
}

The ; is optional before a @ or %, which, according to the docs, "gobbles up everything else".

EDIT: As DVK points out in a comment (and TLP emphasizes in another answer here), you are probably best off simply avoiding prototypes:

sub someFunction {
  my ( $oblig_param1, $oblig_param2, $option_param ) = @_;
  ...
}

Perl prototypes have their uses (mostly to supply implicit context coercion to arguments, as Perl's built-in functions do). They should not be used as a mechanism to check that function are called with the correct number and type of arguments.