I found this (more or less) in Gtk::cookbook (comes with Gtk-Perl... just do man Gtk::cookbook). Sorry, no ideas about "transparent" windows (though I'm not 100% sure what you mean by "transparent").
#!/usr/bin/perl
use warnings;
use strict;
use Gtk '-init';
my $mw = Gtk::Window->new();
my $lb = Gtk::Label->new("This is only a test.");
$mw->border_width(5);
$mw->add($lb);
$mw->signal_connect('key_press_event', sub { Gtk->main_quit; 1 });
###############################
# Note that you have to realize it
# before you can access $mw->window
$mw->realize;
$mw->window->set_decorations(0);
###############################
$mw->show_all;
Gtk->main;
bbfu
Black flowers blossum
Fearless on my breath