Method Signatures 

my $article = Article->new(
    title => "A day at the races",
    body  => "Noddy and Big Ears packed a picnic ...",
);

$article->add_keyword('races');
$article->add_keyword('picnic');
$article->add_keyword('races');

say "Keywords: ", join ', ', @{ $article->keywords };
Keywords: races, picnic