When Perl calls a recursive function it does a huge amount of work saving state, optimizing the next time you call it, etc. For a sobering comparison, compare the implementation above with the following one where I manage a stack explicitly instead of having a stack of function calls: 
sub A {
  my ($x, $y) = @_;
  my @stack = ();
  while (1) {
    if (0 == $x) {
      if (@stack) {
        $y++;
        $x = pop(@stack);
      }
      else {
        return 1 + $y;
      }
    }
    else {
      if (0 == $y) {
        $x--;
        $y = 1;
      }
      else {
        push @stack, $x-1;
        $y--;
      }
    }
  }
}

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In reply to Re (tilly) 3 (faster): Ackerman vs. Perl : KO in 3 rounds by tilly
in thread Ackerman vs. Perl : KO in 3 rounds by John M. Dlugosz

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