# --- user's code
package MyPackg; 
use constant DBG => 0;
 
package main;
 
...

MyPackg::func();
 
...
 
# --- my code
package MyPackg;
sub func {
   print "DEBUGGING" if DBG; # won't be compiled
}

##</code><code>##

{
  package MyPackg; 
  use constant DBG => 0;
}