So each isn't thread safe. That brings us to keys. Is keys uninterruptible, or just the parts that would cause corruption?

Update: No, keys isn't thread safe either.

use strict;
use warnings;
use threads;
use threads::shared;
use feature qw( say );

use constant NUM_THREADS => 10;

my %h :shared = map { $_ => 1 } 'a'..'z';
my $size = () = keys(%h);

my $done :shared;
my @threads;
for (1..NUM_THREADS) {
    push @threads, async {
       while (!$done) {
          my $test_size = () = keys(%h);
          if ($test_size != $size) {
             warn("$test_size != $size\n");
             return;
          }
       }
    };
}

sleep(10);
$done = 1;
$_->join() for @threads;
[download]

11 != 26
42 != 26
15 != 26
38 != 26
13 != 26
41 != 26
15 != 26
41 != 26
13 != 26
45 != 26
[download]