use URI;
my $user_url = "http://www.stonehenge.com/cgi/dilbert";
my $params = [ max => 5, scary => "%&foo;%" ];
# ...
my $uri = URI->new($user_url);
$uri->query_form(@$params);
print $uri->as_string;

##</code><code>##

http://www.stonehenge.com/cgi/dilbert?max=5&scary=%25%26foo%3B%25