my $ticket = get_ticket($rest_client, $sdn_controller); 

if (! defined $ticket) { 
    print "Could not get ticket from $sdn_controller\n"; 
    exit; 
}

more code .....

sub get_ticket { 

    my ($rest_client, $sdn_controller) = @_; 
    my $ticket;
     ........;

    return $ticket;

}