http://perldoc.perl.org/perlintro.html#Writing-subroutines

Default parameters

http://modernperlbooks.com/books/modern_perl_2014/04-perl-operators.html#TG9naWNhbE9wZXJhdG9ycw

sub function
{
    my ($var_1, $var_2) = @_;

    $var_2 = $var_2 // 42; 

    say $var_1;
    say $var_2;
}

function('test', '84');