$ cat s1.pl
package XXX;
BEGIN { require mro; mro::set_mro( "XXX", 'c3' ); }
use parent qw( File::Find );
use parent qw( File::Find );
1;

$ perl s1.pl 
Inconsistent hierarchy during C3 merge of class 'XXX':
	current merge results [
		XXX,
	]
	merging failed on 'File::Find' at /usr/share/perl/5.14/parent.pm line 26.
BEGIN failed--compilation aborted at s1.pl line 4.