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join _ [] = [] -- this means that join on the empty list is the empty 
+string
join delim strings = foldl1 (\left right -> left ++ delim ++ right ) s
+trings -- this is join implemented with reduce
...

-- or with autocurrying fun
join = foldl1 . ((++) .) . flip (++)

sub join {
    my ( $delim, @strings ) = @_;
    reduce { $a . $delim . $b } @strings;
}
but in this case the concatenation operator is not used directly as th
+e a curried higher order function