RosettaCodeData/Task/Ackermann-function/Perl/ackermann-function-2.pl

sub A {
    my ($m, $n) = @_;
    if    ($m == 0) { $n + 1 }
    elsif ($n == 0) { A($m - 1, 1) }
    else            { A($m - 1, A($m, $n - 1)) }
}