Because I was trying a variety of things, the solution below is rather untidy (stuff left over from things that didn't work)

However, I am running short of time, so I simply give you something that might be what you are looking for.

Main points:
1. Use $mainWindow->update() within loops. It keeps Tk alive
2. Launching process using after allows you to continue processing within your current process (kinda like thread?)
3. If you are using a system command that blocks your process, you should look at fileevent and pipes.

Good luck.

use strict; 
use warnings; 
use Tk; 
my $excelflag;
 
my $mainWindow = MainWindow->new; 
my $textBox = $mainWindow->Scrolled("Text", -width=>80, -height=>7, 
      -scrollbars=>"ose", -wrap=>"word")->pack(); 
 
#my $goBut = $mainWindow->Button(-foreground=>"blue", -text=>"Click", 
#                   -command=>sub { $thr = threads->new(\&excecute) } 
+)->pack(); 
 
my $goBut = $mainWindow->Button(-foreground=>"blue", -text=>"Click", 
                   -command=>sub { excecute() } )->pack(); 

my $stopBut = $mainWindow->Button(-foreground=>"red", -text=>"Stop", 
                   -command=>sub{ $excelflag = 1 })->pack(); 
 
my $exitBut = $mainWindow->Button(-foreground=>"red", -text=>"Exit", 
                   -command=>sub{ $excelflag = 1; exit })->pack(); 
 
MainLoop; 
 
sub excecute 
{ 
        $goBut->configure(-state=>"disabled"); 
        $textBox->configure(-state=>"normal"); 
        $excelflag = 0;
        $textBox->after(2,\&excel);
        open(FILE,"C:/file.txt") or die "Can't open file.\n"; 
        while(<FILE>) 
        { 
                my $message = $_; 
                $textBox->insert('end', $message); 
        } 
        close(FILE); 
 
        $textBox->configure(-state=>"disabled"); 
        $textBox->see('end'); 
        return;
}

sub excel {
        use Win32::OLE; 
        use Win32::OLE::Const 'Microsoft Excel'; 
 
        my ($Excel,$Workbook,$fileOutput,$CurrentSheet,$Range); 
        $fileOutput = "C:/test.xls"; 
 
        $Excel = Win32::OLE->new('Excel.Application', 'Quit') || 
        die "Can't create Excel object. \n"; 
        $Excel->{'Visible'}                  = 1;    #0 is hidden, 1 i
+s visible 
        $Excel->{DisplayAlerts}              = 0;    #0 is hide alerts
+ 
        $Excel->{SheetsInNewWorkbook}        = 1; 
        $Workbook = $Excel->Workbooks->Add(); 
 
        $Workbook->SaveAs($fileOutput) or die "Can't save Excel.\n"; 
        $CurrentSheet = $Workbook->Worksheets(1); 
        $CurrentSheet->Select; 
 
        for (my $i=1; $i<=6500; $i++) 
        { 
                $Range = $CurrentSheet->Range("A$i"); 
                $Range->{Value} = $i; 
                sleep(0.01); 
                $mainWindow->update();
                last if $excelflag;
        } 
 
        $Workbook->Save(); 
        $Workbook->Close(); 
        $Excel->Quit(); 
        Win32::OLE->FreeUnusedLibraries(); 
        $goBut->configure(-state=>"normal");
        return;
}
[download]

Sandy