$main = 'http://cgi3.ebay.com/aw-cgi/eBayISAPI.dll'; 
$ru = "http://encode_this/";
use URI;
my $uri = URI->new($main);
$uri->query_form(
  qw(MfcISAPICommand SignInWelcome userid userid pass pass keepMeSignInOption 1),
  ru => $ru
 );
print $uri->as_string;

##</code><code>##

http://cgi3.ebay.com/aw-cgi/eBayISAPI.dll?MfcISAPICommand=SignInWelcome&userid=userid&pass=pass&keepMeSignInOption=1&ru=http%3A%2F%2Fencode_this%2F