The first thing one must realize is that BEGIN is a special subroutine. sub BEGIN { ... }. But for certain subroutines (BEGIN END CHECK INIT DESTROY, did I miss any?) no sub keyword is needed. Now it's clear that one can't write DEBUG && sub BEGIN { ... } and expect it to be sane. But yes, it does indeed compile. This is when you come to hate indirect object syntax even more.

The indirect syntax allows a block as it's first argument (so you can resolve the object, look at the documentation for print.) This means you can write method BLOCK and optionally have some arguments. Now, in DEBUG && ... the dots are expected to be an expression, not a sub declaration. So perl does what you asked it to: it parses an expression. BEGIN { ... } is an expression. It's indirect object syntax, and BEGIN is the method. do { ... } -> BEGIN Tricky eh?

Now when we've got that straight we need to do what we want in another way. As already said you should use require and import instead of use. And since a BEGIN is a subroutine we can return from it.

use constant DEBUG => 1;
BEGIN {
  return unless DEBUG;
  require CGI::Carp;
  import  CGI::Carp qw/ fatalsToBrowser /;
}
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Cheers,
-Anomo