You probably can use Gtk2::Webkit, as shown in Perl Web Browser using Gtk2 and WebKit. The problem with automating it is to detect when the page is fully loaded before taking your screenshot. To solve that problem, you can setup callbacks on certain available signals, as shown in Another YouTube Video Downloader using GtkWebkit:

my $view = Gtk2::WebKit::WebView->new;
$view->signal_connect( 'notify::progress' => \&notify_progress, undef 
+);
$view->signal_connect( 'load_finished' => \&load_finished, undef );
[download]

Gtk2 can make PDF screenshots.

By the way, Gtk2::Webkit's capabilities have been greatly enhanced by the recent addition of Glib::Introspection. You might want to ask on the Perl/Gtk2 maillist on how to use Introspection, or whether it could help in your case.