Documentation for infix :

Documentation for infix :, assembled from the following types:

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.