The "keywords" method seems to be coded to ignore the parameter, causing the behaviour you see:

sub keywords {
    my $self = shift;
    croak 'Not called as a method by parent object'
        unless ref $self && UNIVERSAL::isa($self, __PACKAGE__);

    my %keywords;
    my $stor = $objstore->{_refaddr($self)};
    for my $handle (@{$stor->{parsed}->{handles}}) {
        for my $keyword ($handle->keywords) {
            $keywords{$keyword} = 1;
        }
    }

    return sort(keys(%keywords));
}
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Since the documentation is not explicit about what it SHOULD do, it is not technically a bug. But I would call it a poor example.

I would recommend sub-classing it to do what you ned, and try to contact the author.