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

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

use Readonly;
Readonly $DEBUG => 0;
...
} else {
    print "Hello, world!\n";
}

BEGIN { $^W = 1; }
use Readonly;
...
    print "Hello, world!\n";
}
test.pl syntax OK