>>605 Perl5

use feature qw{signatures say}; no warnings 'experimental';
sub A($m, $n, $l, $r) {
　say "${l}A($m, $n)$r";
　$m ? $n
　　? A($m-1, A($m, $n-1, "${l}A(".($m-1).', ', ")$r"), "${l}A(".$m.', ', ")$r")
　　: A($m-1, 1, "${l}A(".($m-1).', ', ")$r") : $n + 1;
}
say A(2, 1, '', '');


実行結果
~ $ perl 18_605_Ackermann_func.pl
A(2, 1)
A(1, A(2, 0))
A(1, A(1, A(1, 1)))
A(1, A(1, A(0, A(1, 0))))
A(1, A(1, A(0, A(0, A(0, 1)))))
A(1, A(1, A(1, A(0, 2))))
A(2, A(1, 3))
A(2, A(0, A(1, 2)))
A(2, A(0, A(0, A(1, 1))))
A(2, A(0, A(0, A(0, A(1, 0)))))
A(2, A(0, A(0, A(0, A(0, A(0, 1))))))
A(2, A(0, A(0, A(1, A(0, 2)))))
A(2, A(0, A(1, A(0, 3))))
A(2, A(1, A(0, 4)))
5