First of all, Tk and threads need special handling, which may confound a beginner. You can find alot of sample code with a google for "Perl/Tk threads". However, I will tell you that even with a thread, you will still need to run a timer to update the shared variable in the main Tk thread. So I will skip the thread approach and show a linux example which just uses a timer to repeatedly read the serial port. This example is for a Linux system, but you should be able to adapt it to Win32.

The basic idea, is to set a textvariable $input which automatically gets updated from the repeater.

use warnings;
use strict;
use Tk;
use Device::SerialPort;
 
#my $serial_port = "/dev/ttyS0";
my $serial_port = new Device::SerialPort ("/dev/ttyS0") || die "can't
open COM1\n";

#Setup Perl/Tk
my $mw = MainWindow->new();
$mw->title("Scale");

#Close button
my $button = $mw->Button(
         -text => 'Close',
         -command => sub {$mw->destroy}  # exit is better here
         )->pack(
                 -pady => 20,
                 -side => 'bottom'
         );

 my $title_label = $mw->Label(
         -text => "Weigh-Tronix"
         )->pack(
                 -side => 'left');

 my $input = '---------------';
 my $display_label = $mw->Label(
         -textvariable => \$input,
         -width=>15,  #keep the width stable at 15
                    )->pack(
          -side => 'left');

 # every 10 millisecs update from port
 $mw->repeat(10, sub{
       # $input = $port->input;
       #or maybe better
       if(defined $port->input){ $input = $port->input  }

 });

MainLoop;
 __END__
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