>perl -MO=Deparse -e "$DEBUG = 0; print(qq{debug\n}) if $DEBUG; print(qq{end\n})"
$DEBUG = 0;
print "debug\n" if $DEBUG;
print "end\n";
-e syntax OK

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