Documentation for infix ⊈ assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) infix ⊈ §

multi sub infix:<⊈>($a,$b --> Bool:D)

Not a subset of nor equal to operator.

Returns True if $a is not a subset of $b. Equivalent to !(<=).

say (1,2,3) ⊈ (2,3,1); # OUTPUT: «False␤» 
say (2,3) ⊈ (2,3,1);   # OUTPUT: «False␤» 
say 4 !(<=) (1,2,3);   # OUTPUT: «True␤» 

Language documentation: Sets, bags, and mixes §

From Sets, bags, and mixes

(Sets, bags, and mixes) infix ⊈ §

Returns True if $a is not a subset nor equal to $b, else False. More information, Wikipedia definition.