use WWW::Mechanize::PhantomJS;
my $mech = WWW::Mechanize::PhantomJS->new();
$mech->get('http://google.com');

$mech->eval_in_page('alert("Hello PhantomJS")');
my $png = $mech->content_as_png();

print $png;  # I added this line so the PNG image is sent to STDOUT