Then you asked the wrong question.

Even if there were an alternative implemetation of threading to threads, it wouldn't help because Tk objects are not thread-safe.

Indeed. most guis use a queuing mechanism to allow background threads to interact with the gui thread (singular), and that is an efficient and relatively easy thing to do using threads and Thread::Queue.

Thanks. But it seems Thread::XXX are for 5005 thread. Does it work well with threads?

Yes. Thread::Queue works perfectly in conjunction with threads, despite that the naming convention suggests otherwise (something I've bemoaned myself on a few occasions). By way of demonstration, here's a somewhat trivial tk app that starts two worker threads each updating a different gui element via a single queue:

#!perl -slw
use strict;
use threads;
use Thread::Queue;

## A shared var to communicate progess between work thread and TK
my $Q = new Thread::Queue;

sub work{
    my( $id, $delay ) = @_;
    open PROC,
    qq[ perl -le"\$|=1; print and select('','','', $delay ) for 1 .. 1
+00" |]
        or die $!;
    while( <PROC> ) {
        $Q->enqueue( "$id:$_" );
    }
    close PROC;
}

threads->new( \&work, $_, 0.1 * $_ )->detach for 1 .. 2;

## For lowest memory consumption require (not use)
## Tk::* after you've started the work thread.
require Tk::ProgressBar;

my $mw = MainWindow->new;
my $pb1 = $mw->ProgressBar()->pack();
my $lb  = $mw->Label( -height => 2 )->pack;
my $pb2 = $mw->ProgressBar()->pack();

my $repeat;
$repeat = $mw->repeat( 100 => sub {
    while( $Q->pending ) {
        my( $id, $progress ) = split ':', $Q->dequeue;
        return unless $progress;
        ( $id == 1 ? $pb1 : $pb2 )->value( $progress );
        if( $id == 2 && $progress == 100 ) {
            $repeat->cancel;
            $mw->exit;
        }
    }
} );


$mw->MainLoop;
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