#!/usr/bin/perl

use strict;
use threads;
use threads::shared;

my %outer :shared;

$outer{inner} = &share({});
%{$outer{inner}} = map { $_ => $_ } 'a'..'z';

#################################################################

print "By taking a reference first...\n";

my $iter = 0;
{
    my $ref = $outer{inner};
    while ($iter++ < 10)
    {
        last unless (my ($k, $v) = each %$ref);
        print "$k\n";
    }
}

print "...and so on\n\n";

#################################################################

print "By de-ref the shared hash-of-hashes each time...\n";

$iter = 0;
while ($iter++ < 10)
{
    last unless (my ($k, $v) = each %{$outer{inner}});
    print "$k\n";
}

print "...and so on\n";
[download]

By taking a reference first...
w
r
a
x
d
j
y
u
k
h
...and so on

By de-ref the shared hash-of-hashes each time...
w
w
w
w
w
w
w
w
w
w
...and so on
[download]