I have to agree, with marto here. You are saying you are getting a proxy error and you asking for help with credentials. Seems like two seperate issues. if you are positive that it is a credentials issues, here is a snippet that works for me,

my $server  = 'www.yourhosthere.com:';
my $port_no = 443;
my $realm   = 'Restricted Area'; # Make sure the text is an exact matc
+h
my $user    = 'username';
my $passwd  = 'password';

$ua->credentials( $server . $port_no, $realm, $user, $passwd );
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