The submethod BUILD is (indirectly) called by .bless. It is meant to set private and public attributes of a class and receives all names attributes passed into .bless. The default constructor .new defined in Mu is the method that invokes it. Given that public accessor methods are not available in BUILD, you must use private attribute notation instead.

C&P, straight from the docs

class C {
    has $.attr;
    submethod BUILD (:$attr = 42) {
        $!attr = $attr
    };
};
 
C.new.say; C.new( attr => 666 );
# 42
# 666
[download]

Edit:
The difference between BUILD and TWEAK (as mentioned by anonymous monk below) is, that the former happens while the object is constructed, whereas TWEAK happens after that, when the object has been fully constructed.

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