use strict; 
sub yes { 
        print shift; 
        my $ans = <STDIN>; 
        return $ans =~ /^[yY]/; 
} 
 
sub animal { 
        my $tree = { cargo=>'bird', left=>'', right=>'' };     
        # loop until the user quits 
        while (1) { 
                last unless yes "\nAre you thinking of an animal? "; 
         
                # walk the tree 
                while($tree->{left}) { 
                    if (yes "$tree->{cargo} ? " ) { $tree = $tree->{right}; } 
                    else { $tree = $tree->{left}; } 
                } 
         
                # make a guess 
                my $guess = $tree->{cargo}; 
                if (yes "Is it a $guess? ") { 
                    print "I rule!"; 
                    next; 
                } 
         
                # get new information 
                print "What is the animal's name? "; 
                my $animal = <STDIN>; 
                chomp($animal); 
                print "What question would distinguish a $animal from a $guess? "; 
                my $question = <STDIN>; 
                chomp($question); 
         
                # add new information to the tree 
                $tree->{cargo} = $question; 
                if (yes "If the animal were $animal the answer would be? ") { 
                    $tree->{left} = { cargo=>$guess, left=>'', right=>'' }; 
                    $tree->{right} = { cargo=>$animal, left=>'', right=>'' }; 
                } 
                else { 
                    $tree->{left} = { cargo=>$animal, left=>'', right=>'' }; 
                    $tree->{right} = { cargo=>$guess, left=>'', right=>'' }; 
                } 
                 
        } 
} 
animal;