G'day ostra,

My guess would be that "check_required items" is supposed to be "check_required_items". Note the second underscore ('_') and that this occurs in two places ("&check_required items ..." and "sub check_required items ...").

You haven't indicated which edition of the book you're referring to (I believe there's quite a few). You haven't provided a link to the book or its example source code (if such exists). Consequently, I'm not in position to check this for you.

It may be a typographical error. It might just be a printing error/defect (look closely: are there possibly faint underscores where I indicated). Also, check online for any published errata.

Finally, what you've "copied" is probably not the same as what you see in the book: I imagine it has at least some reasonable indentation. Unfortunately, this brings into question how close what you typed is to the original: other than whitespace it might be identical; however, my guess at a correction involves whitespace.