package xx;
sub new { bless {}, shift }
sub instance { __PACKAGE__->new }
sub print    { print "XX" }

package yy;
@ISA = qw/xx/;
sub print { print "YY" }

package main;
my $yy = yy->instance;
$yy->print;
my $yyy = yy->new;
$yyy->print