The problem is caused because you are (implicitly) trying to share objects across threads which isn't possible.

However, that doesn't stop you from doing what you are trying to do. It just means that you must adapt your programing style to account for it.

As I understand you, you want to be able to write to a 'file', represented by an IO:String object on one thread and then access the 'output', the string, in another thread. That can be done this way.

#! perl -slw
use strict;
use threads;
use threads::shared;

my $var : shared;

sub Update{
    require 'IO/String.pm';
    my $io = IO::String->new( $var ); 
    $io->print( 'Hello world' );
    $io->print( $_ ) for 1 .. 10;
};

my $th=threads->new( \&Update ); 
$th->join();

print "'$var'";
__END__
P:\test>368516
'Hello world
1
2
3
4
5
6
7
8
9
10
'
[download]

There are a couple of things to note here:

1. The IO::String object is not shared between threads.
2. The scalar $var, is shared.
3. The IOString object $io is created inside the thread that will use it and is only called from within that thread.
4. I have required the module into the thread that will use it rather than useing it. This is an optimisation. It saves all the code from IO::String being duplicated into every thread created by the program.

   This isn't strictly necessary. You could use IO::String; in the normal way provided any IO::String objects you create are only called from the thread in which the were created.

That said, if you have a choice, there are better ways of sharing data between threads. If you need a filehandle because you are passing this to some other module for it to write to, then you might be better off using a socket rather than an in-memory file this way.