Documentation for infix : assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) infix : §

Used as an argument separator just like infix , and marks the argument to its left as the invocant. That turns what would otherwise be a function call into a method call.

substr('abc': 1);       # same as 'abc'.substr(1)

Infix : is only allowed after the first argument of a non-method call. In other positions, it's a syntax error.