From language/operators

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multi sub infix:<X>(+lol, :&with! --> Seq:D)
multi sub infix:<X>(+lol --> Seq:D)

Creates a cross product from all the lists, ordered so that the rightmost elements vary most rapidly, and returns a Seq:

1..3 X <a b c> X 9
# produces ((1 a 9) (1 b 9) (1 c 9) 
#           (2 a 9) (2 b 9) (2 c 9) 
#           (3 a 9) (3 b 9) (3 c 9))

The X operator also exists as a metaoperator, in which case the inner lists are replaced by the value from applying the operator to the list:

1..3 X~ <a b c> X~ 9
# produces (1a9 1b9 1c9 2a9 2b9 2c9 3a9 3b9 3c9)