Here is my try. It adds numbers, but is not limited to two arguments. It's recursive and probably not very efficient. (Anyone want to benchmark the two solutions?)

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;
}
[download]

Proof that it works:

$\ = "\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 wo
+rk";
[download]

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