sub add {
    my $current = 0;
    return '' unless @_ = grep length, @_;
    $current += substr $_, -1, 1, '' for @_;
    push @_, substr $current, 0, -1, '' if length $current > 1;
    return &add . $current;
}

##</code><code>##

$\ = "\n";

print add(
    '999999999999999999999997',
                           '1',
                           '2'
);

print add(
    '9999999999',
    '9999999999',
     '999999999',
           '999',
             '9'
);

# Next line is not -w safe: "Deep recursion"... grin.
print add('9' x 5000, 1) eq '1' . ('0' x 5000) ? "Works" : "Doesn't work";