After that statement, $self->{lib} contains the first element of the list returned by $lib->get_list('library').

sub get_list {
    ...
    push @list, $item if ($item->is(@keywords));
    ...
    return @list;
}
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You should definitely invest in a test.pl file. Then you can paste these things into it and Try It And See. Sprinkle lots of print statements so you can follow along the code path wherever it is not clear to you.