perl -MO=Deparse -e"use constant DEBUG=>0; if( DEBUG ) { print 'hello'; }"
use constant ('DEBUG', 0);
'???';
-e syntax OK

perl -MO=Deparse -e"use constant DEBUG=>1; if( DEBUG ) { print 'hello'; }"
use constant ('DEBUG', 1);
do {
    print 'hello'
};
-e syntax OK