You mentioned Win32::OLE but also mention "WMI" in your title. If you're using an external program, how about 'query':

VinsWorldcom@C:\Users\VinsWorldcom> which query
C:\Windows\system32\query.exe

VinsWorldcom@C:\Users\VinsWorldcom> query /?
QUERY { PROCESS | SESSION | TERMSERVER | USER }

VinsWorldcom@C:\Users\VinsWorldcom> query user /?
Display information about users logged on to the system.

QUERY USER [username | sessionname | sessionid] [/SERVER:servername]

  username            Identifies the username.
  sessionname         Identifies the session named sessionname.
  sessionid           Identifies the session with ID sessionid.
  /SERVER:servername  The server to be queried (default is current).

VinsWorldcom@C:\Users\VinsWorldcom> query user
 USERNAME              SESSIONNAME        ID  STATE   IDLE TIME  LOGON
+ TIME
>VinsWorldcom          console             1  Active      none   5/21/
+2013 7:03 PM
[download]

Not sure if the 'query' command functionality is in Win32::OLE.

Also note that the command 'net user USERNAME' returns the last logon time, but it is reset even if the user locks the screen and then unlocks it.