What are good interactive GUI builder packages for Perl?

Gtk2 has glade2 which can write out an XML file usable by Gtk2::GladeXML. Here is an example of how to bundle the XML with the app in the same file.

I misread the question at first. I thought you wanted a GUI editor for making GUIs (which is what glade2 is). You can also create GUIs using Gtk2 without glade2:

#!/usr/bin/perl

use strict;
use warnings;
use Gtk2;

Gtk2->init;

my $window = Gtk2::Window->new;
my $vbox   = Gtk2::VBox->new;
my $label  = Gtk2::Label->new("Hello World");
my $button = Gtk2::Button->new("Press me");

$window->add($vbox);
$vbox->add($label);
$vbox->add($button);

$window->set_default_size(200, 200);
$window->signal_connect(
    destroy => sub {
        Gtk2->main_quit;
    }
);

my $i = 0;
$button->signal_connect(
    clicked => sub {
        $label->set_text("button pressed " . ++$i . " times");
    }
);

$window->show_all;

Gtk2->main;

Try wxPerl!

From the web site:

wxPerl is an extension module allowing the creation of GUI (Graphical User Interface) from Perl; it is built as a wrapper for the awesome wxWidgets C++ GUI toolkit.