If you're looking for any mouse movement, you could also just bind to the "<Motion>" event.  That would simplify your code even more:

use Tk;

use strict;
use warnings;

my $mw = MainWindow->new;
$mw->bind('<KeyPress>', \&keyPress);
$mw->bind('<Motion>',   \&mouseMotion);
MainLoop;

sub quit {
   print "User triggered event -- exiting!\n";
   exit;
}

sub keyPress {
   print "Debug:  keypress occurred\n";
   quit();
}

sub mouseMotion {
   print "Debug:  mouse motion occurred\n";
   quit();
}
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