use constant DEBUG => 0;
if(DEBUG) {
    print "This will be gone.\n";
} else {
    print "Hello, world!\n";
}

##</code><code>##

BEGIN { $^W = 1; }
use constant ('DEBUG', 0);
do {
    print "Hello, world!\n"
};
test.pl syntax OK

##</code><code>##

use Readonly;
Readonly $DEBUG => 0;
if($DEBUG) {
    print "This will be gone.\n";
} else {
    print "Hello, world!\n";
}

##</code><code>##

BEGIN { $^W = 1; }
use Readonly;
Readonly $DEBUG, 0;
if ($DEBUG) {
    print "This will be gone.\n";
}
else {
    print "Hello, world!\n";
}
test.pl syntax OK