use WWW::Mechanize::Firefox;
use File::Spec;

my $mech = WWW::Mechanize::Firefox->new(
  bufsize => 10_000_000, # PNGs might become huge
  autoclose => false
);
$mech->get('http://layout.jquery-dev.net/demos/simple.html');

  my $png = $mech->content_as_png();
  my @links = $mech->by_id('center');
  my $png2 = $mech->element_as_png(@links);
  open my $fh, '>', 'page.png'
      or die "Couldn't save to 'page.png': $!";
  binmode $fh;
  print {$fh} $png;
  close $fh;