I proposed another solution to get a "silent" constructor in my Dog $spot;. It involves merely naming the constructor MODIFY_SCALAR_ATTRIBUTES(). It almost works, but some dummy attribute must be cited for the constructor to be called,

use Some::Other::Class;

my Some::Other::Class $obj :dummy;
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MODIFY_SCALAR_ATTRIBUTES() could be written to take the constructor arguments, %options, from the attribute list, but that feels kind of dirty to me.