As far as I remember SOAP::Lite, the problem is that ->fault is the method of the SOAP::SOM (soap message) class, while your $srv->login goes a step further and calls $som->result internally to extract service specific value from $som (see generate_stub in SOAP::Lite). To get to the fault you can try:

# either
$srv->want_som(1);
my $som = $srv->login(@params);
# or
my $som = $srv->call('login', @params);

if ($som->fault){
}
else {
  my $id = $som->result;
}
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or you can play with the $srv->on_fault property.