You'll probably want to use the put() method and send the appropriate escape sequence. You don't say what type of computer you're trying to connect to, so it's kind of hard to give any specific advice. If you're sending to a PC you'll probably want to send the ANSI escape sequence for PgDn. If you're sending to a *nix box you'll need to know what terminal emulation the remote host is using and send the corresponding sequence. You can build the sequence using Perl's chr() function.
P.S. my use of Net::Telnet has not required this so my advice is mostly speculation. Correct me please if I'm totally off base.