package A;

sub new { return bless {}, shift };

sub foo { return 'A' }

package B1;

use base 'A';

sub foo { return 'B1' }

package B2;

use base 'A';

sub foo { return 'B2' }

package C;

use base 'B1', 'B2';

package main;

my $obj = C->new();

print $obj->foo(), "\n";