sub S{ return [shift]; }
sub P{ return (shift)->[0]; }
sub add{
   my($x,$y) = @_;
   return $x unless $y;
   return add(S($x),P($y));
}
sub arabic{
  my $x = shift;
  return 0 unless $x;
  return 1+arabic(P($x));
}
$y=$x = S S;
print arabic($x),"+",arabic($y),"=",arabic(add($x,$y)),"\n";
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