Documentation for infix βˆ‰ assembled from the following pages:

Language documentation: Operators Β§

From Operators

(Operators) infix βˆ‰ Β§

multi sub infix:<βˆ‰>($a,$b --> Bool:D)

Non-membership operator.

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

say 4 βˆ‰ (123);       # OUTPUT: Β«True␀» 
say 2 !(elem) (123); # OUTPUT: Β«False␀» 

Language documentation: Sets, bags, and mixes Β§

From Sets, bags, and mixes

(Sets, bags, and mixes) infix βˆ‰ Β§

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