Well, the answer is not to use a Tk::messageBox. {grin}

You can create a new Toplevel, populate it, and tell it to destroy itself after a time passes. For example (untested):

my $box = $mw->Toplevel;
my $message = $box->ROText->pack;
$message->insert(end => "Hey there!  Gotta go!");
$box->after(3000, sub { shift->destroy });
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