Just remove the :shared attribute from those variables and the error will "go away". You only need to declare variables shared if you want to share their contents across threads.

You cannot share objects across threads, (which is what the error is telling you), so there is no purpose in making objects shared.

Crap... that pretty much kills the design of this program, as I was passing that %siteHash variable between threads, which contained a URI object. Will Perl ever support something like that? Or is "true" multithreaded applications pretty much out of the question?

Or is "true" multithreaded applications pretty much out of the question?

It's perfectly possible to create multithreaded applications. It's not the threading that is biting you, it's the sharing of objects.

For reasons of architecture which are too boring to repeat, you cannot (directly, and you don't want to know about the indirect mechanisms), share objects between threads. You can share data--that is scalars, hashes and arrays. The trick is to pass data between the threads and create objects within threads--for the exclusive use of that thread.

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

You could pass the URI as a string instead of as an object.

Since it looks like you are trying to create a multi-threaded spider, here is a crude one to get you started. It is simple, it doesn't actually store what it fetches anywhere and could be much cleverer about making links absolute etc., but it demonstrates a few ideas.

Usage:

perl -s script.pl [-THREADS=2] url [url...]
[download]

Use ^C to have it abort cleanly and produce some simple stats about what it did.

#! perl -slw
use strict;
use threads;
use threads::shared;
use Thread::Queue;
use LWP::UserAgent;
use HTML::LinkExtor;
use URI;

$|=1;

our $THREADS ||= 2;

sub fetcher {
    my $tid = threads->self->tid;

    my( $Qwork, $hashref ) = @_;

    my $ua = new LWP::UserAgent;
    my $p = new HTML::LinkExtor;

    while( my $next = $Qwork->dequeue ) {
        next if exists $hashref->{ $next };

        my $res = $ua->get( $next );
        $hashref->{ $next } = $res->code;
        print "$tid: Failed to fetch $next", next unless $res->is_succ
+ess;

        $p->parse( $res->content );
        print "Fetched $next";

        my @links = map{
            my( $tag, %attrs ) = @$_;
            $tag eq 'a'
                ? $attrs{ href }
                : ()
        } $p->links;

        printf "$tid: Stacked %d links\n", scalar @links;
        $Qwork->enqueue( @links );
    }
}

## A place to store the urls visited.
## Checked by all threads
my %urls : shared;

## A queue of links to fetch and follow
## Added to and read by all threads
my $Qwork = new Thread::Queue;

## Prime the pump
$Qwork->enqueue( @ARGV );

## Start the fetchers
my @fetchers = map{ 
    threads->create( \&fetcher, $Qwork, \%urls ) 
} 1 .. $THREADS;

## A flag to abort the main thread and clean up
my $stop = 0;

## ^C aborts 
$SIG{INT} = sub{
    ## Empty the work queue
    $Qwork->dequeue while $Qwork->pending;
    
    ## And stack an undef for each fetcher to abort their main loops
    $Qwork->enqueue( (undef) x @fetchers );
    $stop = 1;
};

## Log some progress information
printf "Fetched: %d  (in Q: %d)\n", scalar keys %urls, $Qwork->pending
+ 
    while not $stop and sleep 5;

## Wait for the fetchers to stop
$_->join for @fetchers;

## Gen some stats
my( %stats, $total );
++$total, ++$stats{ $_ } for values %urls;

## And print them
print "Atempted: $total";
print "Status: $_ ( $stats{ $_ } )" for sort{$a<=>$b} keys %stats;
[download]

Be careful where you let it loose.

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.