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sub new {
    my $class = shift;
...

    return bless { datum => $value, glob => *value }, $class
}

sub value {
    my $self = shift;
...

    return $self->{glob}           # might work
}

my $d = datum->new(42);
*d =  $d->value();

my $datum = \ $obj->{datum};

# and then
$$datum = ...;

$datum->{foo}=...;
push @$datum,...;