From language/operators

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sub infix:<,>(*@a --> List:D) is assoc<list>

Constructs a higher-order Cool from its arguments.

my @list = :god('Þor'), ['is',"mighty"];
say @list;      # OUTPUT: «[god => Þor [is mighty]]␤» 
my %hash = :god('Þor'), :is("mighty");
say %hash.raku; # OUTPUT: «{:god("Þor"), :is("mighty")}␤» 
my %a = :11a, :22b;
say %(%a, :33x);  # OUTPUT: «{a => 11, b => 22, x => 33}␤»

In the first case it returns a List, in the second case, since the arguments are Pairs, it builds a Hash.

It can also be used for constructing variables from other variables, collating elements of different types, in this case a Hash and a Pair:

my %features = %hash, :wields("hammer");
say %features;  # OUTPUT: «{god => Þor, is => mighty, wields => hammer}␤» 

The comma is also used syntactically as the separator of arguments in calls.