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$mw->Button(
        -text => "Submit",
        #-command => sub {exit} # no good, will exit whole program
         -command => sub{ $mw->destroy }   # good 
)->pack();

....  Tk code omitted
MainLoop;
...
#get the error from your_command
chomp(my $error = <ERROR>);
print "error = $error\n";