*DEBUG = \1;
use constant CONST_DEBUG => 1;
$DEBUG && print "foo\n";
CONST_DEBUG && print "bar\n";

##</code><code>##

## Output of perl -MO=Deparse on my machine with perl 5.6.1:
*DEBUG=\(1);
sub CONST_DEBUG() {
    package constant;
    $scalar;
}
print "foo\n" if $DEBUG; # <- $DEBUG must be evaluated each time
print "bar\n"; # <- there's no evaluation... just straight print()'ing