[ Ignore this post. I missed the "In the real module" bit initially. I'll post something else in a minute. ]

Close. Change

my $ob1 = foo->new('alpha');
my $ob2 = foo->new('beta');
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to

my $ob1 = alpha->new();
my $ob2 = beta->new();
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And set @ISA for alpha and beta.

use strict;
use warnings;

my $ob1 = alpha->new()
my $ob2 = beta->new()

$ob1->write;
$ob2->write;

BEGIN {
    package foo;
    sub new{
        my $class = shift;
        return bless {}, $class;
    }
    sub write{
        my $self = shift;
        print $self->read;
    }
}

BEGIN {
    package alpha;
    our @ISA = 'foo';
    sub read{
        return "Alpha\n";
    }
}

BEGIN {
    package beta;
    our @ISA = 'foo';
    sub read{
        return "Beta\n";
    }
}
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