• Two common means of passing arguments to subs
• Pass by value
• Pass by reference
• Perl allows either
• Arguments are passed into the @_ array
• @_ is the "fill in the blanks" array
• Usually should copy @_ into local variables

        sub add_one {           # Like pass by value
            my ($n) = @_;       # Copy first argument
            return ($n + 1);    # Return 1 more than argument 
        }

        sub plus_plus {         # Like pass by reference
            $_[0] = $_[0] + 1;  # Modify first argument
        }

        my ($a, $b) = (10, 0);

        add_one($a);            # Return value is lost, nothing changes
        $b = add_one($a);       # $a is 10, $b is 11

        plus_plus($a);          # Return value lost, but a now is 11
        $b = plus_plus($a);     # $a and $b are 12