Documentation for postfix ,= assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) postfix ,= §

Creates an object that concatenates, in a class-dependent way, the contents of the variable on the left hand side and the expression on the right hand side:

my %a = :11a, :22b;
%a ,= :33x;
say %a # OUTPUT: «{a => 11, b => 22, x => 33}␤»