#!/usr/bin/env perl
# Copyright (c) 2015 Christian Jaeger, copying@christianjaeger.ch
# This is free software. See the file COPYING.md that came bundled
# with this file.
use strict;
use warnings;
use warnings FATAL => 'uninitialized';
# "This token is only available under use v5.16 or the "current_sub" feature"
use v5.16;
sub foo {
    my $f = sub {
        my ($n) = @_;
        my $f = __SUB__;
        sub {
            if ($n > 0) {
                $n + &{ &$f($n - 1) }
            } else {
                0
            }
        }
    };
    goto &$f
}
my $res = &{ foo 2 };
print $res, "\n";