Quoting Net::Telnet:

This method returns a scalar reference to the input buffer for $obj. Data in the input buffer is data that has been read from the remote side but has yet to be read by the user.

So, the following use would be correct:

    my $ref= $obj->buffer;
    print "Unread buffer is '$$ref'\n";
[download]

If that is empty, then there is no unread data.