${ *{${ns} . '::foo'} } = $cram;

##</code><code>##

${ $::{$ns.'::'}{foo} } = $cram;

##</code><code>##

#!/usr/bin/perl -l
use strict;
use warnings;

my $ns = 'Teddy::';

${ $::{$ns}{foo} } = 'cheese';
print '$Teddy::foo: ', $Teddy::foo;

# or the other way round
$Teddy::foo = 'bar';
print '${ $::{$ns}{foo} }: ', ${ $::{$ns}{foo} };

__END__
$Teddy::foo: cheese
${ $::{$ns}{foo} }: bar

##</code><code>##

print grep /Teddy/, keys %::;    # Teddy:: 
print $::{'Teddy::'};            # *main::Teddy::
print keys %{$::{'Teddy::'}};    # foo
print $::{'Teddy::'}{foo};       # *Teddy::foo

##</code><code>##

$::{'Teddy::'}{foo} = \'cheese';

print $Teddy::foo;    # cheese
$Teddy::foo = 'bar';  # "Modification of a read-only value attempted at ..."