Creating and calling subroutines

The sub keyword creates a subroutine. A typical subroutine has a name, a list of formal parameters, and a body. However, both the name and the parameter list are optional. In Chapter 1, What is Perl 6?, we already created a subroutine to add two numbers. Let's recall it here:

sub add($x, $y) {
    return $x + $y;
}

Here, add is the name, which will later be used to call a sub. It is followed by a list of the sub's parameters—($x, $y). The body of the subroutine is enclosed inside a pair of curly braces—{return $x + $y;}.

To call a subroutine, use the name again and pass the actual parameters in parentheses:

my $a = 17;
my $b = 71;
my $sum = add($a, $b);
say "Sum of $a and $b is $sum"; # Sum of 17 and 71 is 88

There are two ways a sub can return a value. The first we just saw in the add function. It uses an explicit...