substr uses p5 style "from the end" syntax

[coke]

< [Coke]> pugs: my $str = "foobar"; say substr($str, *-1); <+p6eval> pugs: OUTPUT«foobar␤» < [Coke]> pugs: my $str = "foobar"; say substr($str, -1); <+p6eval> pugs: OUTPUT«r␤»

See t/spec/S32-str/substr.t for tests.