In using the "resizable" method, the first value is the x value, the second the y. So you can control whether or not the user can resize horizontally, vertically, or both.

my $mw = MainWindow->new(-title => "No Resize");
$mw->resizable(0,0);
[download]

If you haven't already, I would highly suggest Mastering Perl/Tk- it's a very informative book and a great reference when build Perl/Tk apps. That's where I got that info from.