Theres no way to find out the prompt before you login, as you need to login to do that :)
Net::Telnets default Prompt is /[\$%#>] $/ which is pretty generic, just wants any line which ends in $, %, k, > and then a single space. Maybe you can try an even more generic regexp (I wonder what .* would do?)

Failing that (and as I'm not particularly enamoured of Net::Telnet for this very reason), you could try either Net::SSH / Net::SSH::Perl if you have ssh on the machines, or just a plain old IO::Socket::INET

C.