Lvalues substr for Rakudo

lsubstr.pl

sub lsubstr($s is rw, $from = 0, $chars = $s.chars - $from) {
    my $substr = substr($s, $from, $chars);
    Proxy.new(
        FETCH   => sub ($) { $substr },
        STORE   => sub ($, $new) {
            $s = $s.substr(0, $from)
               ~ $new
               ~ $s.substr($from + $chars);
        }
    );
}

my $s = 'foo';
lsubstr($s, 0, 1) = 'bar';
say $s;