I doubt that Tkx has a MainWindow object because the MainWindow object is available in Tk but not Tkx. Show us the code that generated this problem because without it we'd just be punching in the dark with regard to what possibly could have caused this as zwon has told you. Further, I'd demonstrate to you a basic difference between these two modules that can be related to your problem.

This is how the Tkx module is invoked (run "perldoc Tkx" from your shell for more info):

use strict;
use warnings;
use Tkx
my $mw=Tkx::widget->new
....
....
....

Tkx::MainLoop;
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And this is how you invoke the Tk module (run "perldoc Tk" from your shell for more info):

use strict;
use warnings;
use Tk;
my $mw=MainWindow->new();
...
...
...
MainLoop;
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Don't forget to check perldoc for each one of these modules, it can potentially give you basic information that could enable you to find where the error is yourself, if you couldn't then post that code here for us please. Have a nice Perl journey..