In Operators§

See primary documentation in context for 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  (123);       # OUTPUT: «True␤» 
say 2 !(elem) (123); # OUTPUT: «False␤» 

In Sets, bags, and mixes§

See primary documentation in context for infix ∉

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