but it doesn't work for me:

Not surprising, $/="\n" assignment is always true (and assignment ), 5.12 is not a module (maybe you wants features), and 5.12 doesn't export say (features you want, maybe)

Apparently, 'use if' can only see a global variable that exists at compile time.

nothing specific to if, that is the nature of compile-time and our/my, initialization doesn't happen until later

use constant DEBUG => !!( 0 || $ENV{PERL_DEBUG_MYAPPNAME} );
use if DEBUG , qw' Carp::Always ';