I agree with the already expressed sentiments about Tk (I've used it a bit and found it useful).
in reply to Is there a really perlish user interface module, or something?
Recently I've been using PerlQt.
The GUI builder (qt designer) is very easy to use and actually works pretty well with Perl.
Here's a program that demonstrates your popup message box:
my $a = Qt::Application();
my $ret = Qt::MessageBox::information(undef, $0,
'Are you sure you want to quit?'