From language/setbagmix

See Original text in context

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

From language/operators

See Original text in context

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.