You can't do threads from within Tk.

However, you could spawn one or more threads outside of Tk, and then use Tk only within one of those threads.

Here's a simplistic example, based on your code, where the "parent" thread runs perl/Tk, and the "worker" thread sends messages back to the parent:

#!/usr/local/bin/perl -w

use Tk;
use threads;
use threads::shared;


#############
## Globals ##
#############
my $shared_text:   shared = "";        # The message sent back to the 
+parent
my $shared_flag:   shared = 0;        # 0 = no msg waiting, 1 = msg wa
+iting


##################
## Main program ##
##################

my $pthread = threads->new(\&worker_thread);
$pthread->detach();

my $mw = MainWindow->new();
my $text = $mw->Entry(-width => 8)->pack();

$text->insert(0, 'abcde');

my $button = $mw->Button(-text=>'click', -command => \&talk_to_worker)
+->pack();
MainLoop;


#################
## Subroutines ##
#################

# Parent subroutines
sub talk_to_worker{
    while (0 == $shared_flag) {
        print "(parent)  Waiting for flag to go to 1\n";
        select(undef, undef, undef, 0.5);
    }

    my $msg = $shared_text;
    print "(parent)  Got message '$msg'\n";

    if ($msg) {
        $text->delete('0.0', "end");
        $text->insert('0.0', $msg);
        $mw->update();
    }

    print "(parent)  Resetting flag to 0\n";
    $shared_flag = 0;
}



# Worker subroutines
sub worker_thread {
    for my $i (0..100) {
        sleep 3;
        send_message_to_parent("-$i");
    }
}


sub send_message_to_parent {
    my ($msg) = @_;

    # Wait until the $shared_flag is zero again
    while (1 == $shared_flag) {
        print "(worker)  Waiting for flag to go to 0\n";
        select(undef, undef, undef, 0.5);
    }

    # Send the message;
    $shared_text = $msg;

    # Raise the flag to handshake with the parent
    $shared_flag = 1;
}
[download]

Note the use of threads::shared, which lets you pass messages between threads.  Also, the use of a flag $shared_flag, which is set by the child thread (the "worker" thread) to indicate that it has sent a message in $shared_text, and then reset to zero by the parent thread to indicate that the message has been received.