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');