use strict;
use warnings;

# Parent class.
package Foo;

sub new {
    my $class = shift;
    my $subclass = shift;
    {
        no strict 'refs';
        push @{"${subclass}::ISA"}, "Foo" unless
            grep {$_ eq "Foo"} @{"${subclass}::ISA"}
    }
    bless {} => $subclass;
}

package Alpha;
sub read {return "Alpha"}

package Beta;
sub read {return "Beta"}

my $oa = Foo->new("Alpha");
my $ob = Foo->new("Beta");