Here's what might be considered a work-around for the limitation:

#! perl -slw
use strict;
use threads;
use threads::shared;


my $some_id = 1;

## Make the scalar shared
my $scalar_data :shared = 'fred';

my %planets :shared;

my %el :shared  = (
    'data' => \$scalar_data, ## reference to shared scalar
    'atime' => time,
    'deleted' => 0,
    'modified' => 0
);

$planets{$some_id} = \%el;

## Lock the shared hash (%el) by indirection
lock %{ $planets{$some_id} };

## Or lock the data itself via indirection
lock ${ $planets{ $some_id }{ data } };
[download]

Seems a tad hookey, but it does allow you to lock the relevant sub-hash or piece of data without requiring you to lock the entire top-level hash.