I notice that you're passing servername rather than ProfileName to the Logon function, when according to my handy ActiveState Win32::OLE Type Library Browser the Logon function for MAPI.Session looks like:

Function Logon([ProfileName], [ProfilePassword], [ShowDialog], [NewSession], [ParentWindow], [NoMail], [ProfileInfo])

I have a mail filter script that uses the following code (which seems to work with your code as well):

$session = Win32::OLE->new("MAPI.Session") 
    or die "Can't establish session : ".Win32::OLE->LastError();
$session->Logon(undef, undef, 0, 0);
[download]

You may have to actually provide values for ProfileName and ProfilePassword depending on how the defaults are set on your system.

-- grummerX