sub DEBUG {0}
print "blabla" if DEBUG;

##</code><code>##

C:\>perl -MO=Deparse -e "print 'this' if $x"
print 'this' if $x;
-e syntax OK
C:\>perl -MO=Deparse -e "print 'this' if 0"
'???';
-e syntax OK

##</code><code>##

C:\>perl -MO=Deparse -we "sub DEBUG{0}print 'this' if DEBUG;"
sub DEBUG {
    0;
}
print 'this' if DEBUG ;
-e syntax OK

C:\>perl -MO=Deparse -we "sub DEBUG(){0}print 'this' if DEBUG;"
sub DEBUG () {
    0;
}
'???';
-e syntax OK