I used IPC::System::Simple. I took a screenshot of CNN, and everything was there.

#!/usr/local/bin/perl

use strict;
use warnings;
use IPC::System::Simple qw(capture);

my $url = 'http://www.cnn.com';
system("firefox", $url); 
sleep 30;
$url =  capture("import -window root MyScreenshot.png");
print $url, "\n";
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