The dialog call is now reduntant. Try this:

use strict;
use warnings;
use threads; use threads::shared;
use Win32::GUI();

my $text :shared;
my @h = qw(LINE1 LINE2 LINE3 LINE4 LINE5);

my $pid = fork();
if ($pid){
    my $main = Win32::GUI::Window->new(
        -left   => 100,
        -top    => 100,
        -width  => 310,
        -height => 150,
    );
    $main->Show();
    my $display = $main->AddTextfield(
        -left   => 60,
        -top    => 35,
        -width  => 180,
        -height => 20,
    );
    while(1){
         $display->Show();
         $display->SelectAll();
         $display->Clear();
#         $display->Update();
         $display->Append($text);
         $display->Update();
         sleep(1);
    }
}
elsif ($pid == 0) {
    while(1){
        $text = "$h[int(rand(@h))]";
        sleep(1);
    }
}
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