perl protocol

===> Expectations

- perl v7 is declared to be perl v5.XX, one of features in bundle will be 'indirect'
  - with feature indirect will be default off
  - enabling of feature indirect will be prohibited in v8

===> Code, running perl-5.XX, module already compatible with v7

use v7;
use feature 'indirect';

# perl-5.XX knows about bundle v7 so this is legal code and use will succeed
# enabling 'indirect' will raise deprecation warning

===> Code, running perl-7, module incompatible with v7

# use v7 is default ("base protocol")
use v5.XX;
use feature 'indirect';

# v5.XX is known protocol to v7
# enabling 'indirect' will raise deprecation warning

==> Code, running perl-8

use v5.XX;

# compile time error, protocol cannot be enabled due 'indirect'

==> Code, running perl-5.XX

use experimental v8; # or "use v8-preview"
use feature 'indirect';

# compile time error, protocol cannot be enabled due 'indirect'
# as far as it's already decided to get rid of feature 'indirect'
# one can write as v8-compatible code as possible