# Bar.pm:
use strict;
use warnings;

package Bar;

=begin DEBUGGING
die 'Bar dies';
=cut

1;
__END__
# foo.pl
use strict;
use warnings;

use Bar;

die 'foo.pl dies';

__END__
% perl foo.pl
foo.pl dies at foo.pl line 6.
% perl -Mifdef=DEBUGGING foo.pl
Bar dies at /loader/0x8128de4/Bar.pm line 7.
Compilation failed in require at foo.pl line 4.
BEGIN failed--compilation aborted at foo.pl line 4.