package base;
use overload
    '""'     => sub { shift->as_string },
#    '""'     => \&as_string,
    fallback => 1;

sub as_string { "this is a base class" }

package child;
@ISA = qw(base);
sub as_string { "this is a child class" }

package main;

$obj = bless {}, "child";

print "\$obj=$obj\n";