there isn't any book on Perl 6 objects yet.

My best bet would be to look at the official documentation on objects (https://docs.perl6.org/language/objects) and on the meta-object protocol (https://docs.perl6.org/language/mop).

I do not know what the before/around/after methods do in CLOS, but, judging from the names, you might be able to implement something similar in Perl 6 using phasers: https://docs.perl6.org/language/phasers. Look especially at block phasers: https://docs.perl6.org/language/phasers#Block_Phasers.