use constant DEBUG => 0;

if (DEBUG)
{
	print "Calculator with debugging...";
}
else
{
	print "Calculator...";
}

print "1 + 1 = ", (1 + 1), "\n"; 

##</code><code>##

use constant ('DEBUG', 0);
do {
    print 'Calculator...'
};
print '1 + 1 = ', 2, "\n";

##</code><code>##

use Readonly;
Readonly my $DEBUG => 0;

if ($DEBUG)
{
	print "Calculator with debugging...";
}
else
{
	print "Calculator...";
}

print "1 + 1 = ", (1 + 1), "\n"; 

##</code><code>##

use Readonly;
Readonly my $DEBUG, 0;
if ($DEBUG) {
    print 'Calculator with debugging...';
}
else {
    print 'Calculator...';
}
print '1 + 1 = ', 2, "\n";