> perl -MO=Deparse tst.pl
sub BEGIN {
    sub MyPackg::DBG {
        0;
    }
    
}
our $a = 1;
package MyPackg;
print 'DEBUGGING' if DBG();  # <-- oops
print 'huhu';
tst.pl syntax OK

##</code><code>##

BEGIN {
  sub MyPackg::Debug {!!0};
}
 
 
our $a=1;
 
package MyPackg;
 
use constant DBG => MyPackg::Debug;
 
print "DEBUGGING" if DBG; # won't be compiled
 
print "huhu";

##</code><code>##

 > perl -MO=Deparse tst.pl
sub BEGIN {
    sub MyPackg::Debug {
        0;
    }
    
}
our $a = 1;
package MyPackg;
sub BEGIN {
    require constant;
    do {
        'constant'->import('DBG', Debug())
    };
}
'???';
print 'huhu';
tst.pl syntax OK