I was messing with something similar yesterday (in trying to adjust the transport, where I was getting the other connection info from a wsdl)

Unfortunately, I had no luck, and resorted to giving up on the wsdl, and going back to specifying the proxy and uri instead.

I was going with a different approach, however, if you'd like to try the logic I was using, but you'll have to rework part of SOAP::Lite:

use SOAP::Lite;
my $soap = SOAP::Lite->service( $wsdl_url );
$soap->transport()->credentials( $hostid, $realm, $user, $pass );
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The problem is that the transport hasn't been fully initialized yet, so there's no LWP::UserAgent (or whatever your transport is) to start fiddling with.

In my case, I had a workaround (place the user/password in the proxy string), so ended up going that route, but it kept me from being able to use the wsdl directly.

To get my logic to work, you'd probably have to find where the transport gets initialized, and force that to run earlier (I'd do it myself, but I'm bogged down with more pressing concerns this week (month?).)