Why do you assign a nicely shared anonymous hash my $href = &share({});, and then immmediately overwrite it with a non-shared one: $href = {};?

What you are doing is assigning a reference to a shared anonymous hash, to a non-shared scalar. You then immediately overwrite it with a reference to a non-shared anonynous hash.

And you do a similar thing again here:

$href->{a} = &share({});
$href->{a} = {};
[download]

It's kind of very surprising to me that anything gets shared this way(*), but if simply remove that bewildering practice, then you'll get something approaching the effect you are looking for:

#!/usr/bin/perl 

use warnings;
use strict;
use threads;
use threads::shared;
use Data::Dumper;

my $href = &share({});
#$href = {};

sub sub1 {
  sleep 1;
  print threads->tid();
  print Dumper $href;
}

$href->{a} = &share({});
#$href->{a} = {};

my $th1 = threads->new('sub1');
$href->{b} = &share({});
my $th2 = threads->new('sub1');

$th1->join();
$th2->join();
__OUTPUTS_
C:\test>junk1
1$VAR1 = {
          'a' => {},
          'b' => {}
        };

2$VAR1 = {
          'a' => {},
          'b' => {}
        };
[download]

That said, I don't understand why you are doing things the way you are doing them. But I realise this is just a snippet, so it might make more sense in your real code. There seems to be a tendancy to use hash refs where a simple hash would do. This is equivalent to the above, far simpler, and more reliable because perl won't let you accidently overwrite the shared hash with a non-shared one as you've been doing with the unshared reference to a shared hash.

#!/usr/bin/perl

use warnings;
use strict;
use threads;
use threads::shared;
use Data::Dumper;

my %hash :shared;

sub sub1 {
  sleep 1;
  print threads->tid();
  print Dumper \%hash;
}

$hash{a} = &share({});

my $th1 = threads->new('sub1');
$hash{b} = &share({});
my $th2 = threads->new('sub1');

$th1->join();
$th2->join();
__OUTPUTS__
C:\test>junk1
1$VAR1 = {
          'a' => {},
          'b' => {}
        };

2$VAR1 = {
          'a' => {},
          'b' => {}
        };
[download]

(*)It would be interesting to see what the p5p guys make of that, cos it confuses the begebbers outta me!