$main->fileevent(\*STDERR,'readable',[\&tackle_err]);

sub tackle_err{
        $_ = <STDERR>;
        my $error_window = $main->Toplevel();
        $error_window -> Label (
                -text => $_
        )->pack;
        $error_window -> Button (
                -text => "OK",
                -command => sub {$error_window->destroy}
        )->pack;
}