one-lin.p6

sub MAIN ($length){say (1...$length).map({("*" x $length).comb.join("_").subst(/\*_/," ", :x($length-$_))}).join("\n")}

recurs.p6

sub step($line) {
    say $line;
    $line ~~ /\s/ && step($line.subst(/\s(\S)/,{"*_$0"}));
}

step(" " x 10 ~ "*");

simple.swift

func arbreDeNoel(n: UInt, indentation: String = "") -> String {
    switch n {
    case 1:
        return printArbre(branche: "*", indentation: indentation)
    default:
        return printArbre(branche: arbreDeNoel(n: n - 1, indentation: indentation + " ") + "_*", indentation: indentation)
    }
}
func printArbre(branche: String, indentation: String) -> String {
    print(indentation + branche)
    return branche
}
arbreDeNoel(n: 10)