Solution: Comma-separated list

Here is a possible solution to the task.

Code

my @words = 'Raku', 'is', 'fun';

say @words.reduce(-> $a, $b { "$a, $b" });

🦋 You can find the source code in the file comma-separated.raku.

Raku, is, fun

Here the block builds a value instead of choosing one: $a is the string assembled so far and $b is the next word, and "$a, $b" glues them with a comma and a space.
The first call joins 'Raku' and 'is' into Raku, is; the second joins that with 'fun' to give Raku, is, fun. This is the kind of separator-aware joining the [~] metaoperator cannot express on its own.
In real code you would reach for the built-in join method, which does exactly this — say @words.join(', '); prints the same Raku, is, fun. The point of the exercise is to practise expressing the same idea as a reduce block.