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$top->Busy;
call_you_sub_here();
$top->Unbusy;

use Tk;
my $mw = MainWindow->new;
$mw->Button(-text => "Goodbye World!", -command =>sub{exit})->pack;
...
            -command =>sub{$mw->Busy; sleep 1;$mw->Unbusy;})
   ->pack;
MainLoop;