Documentation for infix ∉ assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) infix ∉ §

multi sub infix:<∉>($a,$b --> Bool:D)

Non-membership operator.

Returns True if $a is not an element of $b. Equivalent to !(elem).

say 4 ∉ (1, 2, 3);       # OUTPUT: «True␤» 
say 2 !(elem) (1, 2, 3); # OUTPUT: «False␤» 

Language documentation: Sets, bags, and mixes §

From Sets, bags, and mixes

(Sets, bags, and mixes) infix ∉ §

Returns True if $a is not an element of $b, else False. More information, Wikipedia definition.