It is entirely possible to share a multi-level hash between threads. There are a few rules to follow though.

  1. Any value you store in a shared hash must itself be shared (or sharable). In the case of a hashref, this means that the reference must point to a shared hash. But, the share function will not accept an anonymous hash reference.
  2. The hash must be shared before you populate it. If you use the share function on a populated hash, it will be silently emptied.
The combination of these two (strange) design decisions mean that you must declare any subhash as shared (:shared), then populate it. You can then assign a reference to it, as a value in another shared hash.

The following code is not intended as good coding technique, it simply serves to demonstrate that sharing nested hashes is possible.

  • %hash is declared so that thread() closes over it.
  • A thread is spawned and it loops 10 times adding a new HoHs each time around the (sleep delayed for demo purposes) loop.
  • The main thread then goes on to dump the contents of the shared hash in a loop until the demo thread stop. You will see the hash growing in steps. 
    #! perl -slw
use strict;
use threads;
use threads::shared;
use Data::Dumper;

$Data::Dumper::Indent = 0;

my %hash : shared;
my $running : shared = 0;

sub thread {
    $running++;
    for my $mainKey ( 1 .. 10 ) {
        my %subhash : shared = map {
            my %subsubhash : shared = map{;
                ( "foo$_" , "bar$_" )
            } 1 .. 4;
            \%subsubhash;
        } 1 .. 4;
        {
            lock %hash;
            $hash{ $mainKey } = \%subhash;
        }
        sleep 1;
    }
    $running--;
}

my $thread = threads->new( \&thread );
sleep 1 until $running;

while( $running ) {
    print Dumper \%hash;
    sleep 1;
}

$thread->join;

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    In reply to Re: sharing a hash of hashes between threads by BrowserUk
    in thread sharing a hash of hashes between threads by coontie

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