gistfile1.txt

use Ubic::Credentials;

my $current_cr = Ubic::Credentials->current;

my $service_cr = $service->credentials; # or maybe to avoid adding one more method to Ubic::Service interface, Ubic->credentials($service_name)

$guard = $service_cr->effective_guard; # guard 
... # write status file
undef $guard;

$service_cr->activate; # there is no way back from here - you can't change real uid and then change it back