#!/usr/bin/perl -w
use strict;
use warnings;

sub add {
        my ($first, @rest) = @_;

        if (!defined $first) {
                0;
        }
        else {
                die "operand may not be negative!" unless $first >= 0;
                if ($first == 0) {
                        add(@rest);
                }
                else {
                        1 + add($first-1, @rest);
                }
        }
}

sub print_add {
        print join(" + ", @_), " = ", add(@_), "\n";
}

print_add 2,2;
print_add 2,2,2;
print_add 4,8,12;