Das heisst einfach, dass die benutzte Shell keine Login-Shell ist.
Die Dokumentation zu getlogin() zeigt Abhilfe:

[quote= ,perldoc -f getlogin]
getlogin
   Implements the C library function of the same name, which on most systems returns the
   current login from /etc/utmp, if any.  If null, use "getpwuid".

                  $login = getlogin || getpwuid($<) || "Kilroy";

   Do not consider "getlogin" for authentication: it is not as secure as "getpwuid".
[/quote]