your question has already been answered, you have to pass references.

But the code you are showing has too many issues.

this doesn't make sense since $value is localized and will be deleted after new() returns.

sub new {
    my $class = shift;
    local $value = shift;

    return bless { datum => $value, glob => *value }, $class
}
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the only "slot" holding the value is $obj->{datum}

then be aware that $self is supposed to hold an object (i.e. blessed hashref), you have to deref $self properly

sub value {
    my $self = shift;
    
    # return $self{glob}           # will never work

    return $self->{glob}           # might work
}
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finally this is nonsense, you can't use a glob sigil with a private variable my $d

my $d = datum->new(42);
*d =  $d->value();
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your alias must be a package var.

If you really want to go on with shortcuts, why not simply try a scalar ref?

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

# and then
$$datum = ...;
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> # $d->{foo} = ...; push @d, ...

well hashes and arrays must be refs anyway...

so you'll need to write things like

$datum->{foo}=...;
push @$datum,...;
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But plz be aware that you are now committing crimes against many laws of OOP!