My first guess would be, that you give bless the parameters in wrong order: So in package Duck,

sub new {
    # shift means shift @_ and will have $_[0] inside
    # what means $self like in package Animal 
    return bless(shift, {text => "I am a a duck.\n"});
    # and the class name needs to be first parameter
}
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So, you accidently blessed the class 'I am a duck.\n' instead of the class Duck :-) But as 'I am a duck.\n'-class also @ISA Animal, it still could speak and speaks there $self->{text} that had never been defined anywhere, so it prints nothing at all absolutely correctly :D