How do I declare an optional parameter in a signature?

Question

Let's say I have

use v5.026;
use feature 'signatures';

sub foo ($opt1, $opt2) {
  say $opt1 if $opt2;
}

main::foo(1,2);
main::foo(1);

Now I want to call foo with and without opt2:

foo(1);    # not currently accepted
foo(1,2);  # works fine

Answer 1

Optional parameters with subroutine signatures require a defined default which is done with = default_value_expression. You can hard set this to undef:

sub foo ($opt1, $opt2 = undef) {
  say $opt1 if $opt2;
}