Just use URI as it was intended:

$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 keepMeSignI
+nOption 1),
  ru => $ru
 );
print $uri->as_string;
[download]

which generates

http://cgi3.ebay.com/aw-cgi/eBayISAPI.dll?MfcISAPICommand=SignInWelcom
+e&userid=userid&pass=pass&keepMeSignInOption=1&ru=http%3A%2F%2Fencode
+_this%2F
[download]

-- Randal L. Schwartz, Perl hacker