Documentation for infix (elem), infix ∈ assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) infix (elem), infix ∈ §

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

Membership operator.

Returns True if $a is an element of $b.

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

Since release 2020.05, ∊ is an alias for this operator.

Language documentation: Sets, bags, and mixes §

From Sets, bags, and mixes

(Sets, bags, and mixes) infix (elem), infix ∈ §

Returns True if $a is an element of $b, else False. More information, Wikipedia definition.