Here's another approach using Win32::GUI that I haven't seen posted. It's a variation of something I've done in other programming languages, but it seems to work well enough on my desktop running XP. The downside is that it pretty much throws portability out the window.
use Tk;
use Win32::GUI;
my $mw = MainWindow->new;
$mw->title("Hello");
$mw->update;
my ($winH) = Win32::GUI::FindWindow("", "Hello");
my $style = Win32::GUI::GetWindowLong($winH, -16);
$style &= ~(WS_MINIMIZEBOX | WS_MAXIMIZEBOX);
Win32::GUI::SetWindowLong($winH, -16, $style);
Win32::GUI::DrawMenuBar($winH);
MainLoop;