#!/usr/local/bin/perl

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

$|=1;

our $db :shared;

sub func() {
  my $new;

  while (1) {
    print STDERR "func: new iteration started\n";
    sleep(1);
    $new = shared_clone({}); # reference to new shared hash
    $$new{'key'} = shared_clone([1, 2, 3]); # fill the hash
    {
      lock($db);
      $db = $new;
      $new = undef;
    }
    print STDERR "func: iteration finished\n";
  }
}

# parent thread

$db      = shared_clone({});
threads->create(\&func)->detach();
while(1) { sleep(1); }