how do i open a remote file from telnet session?

At first I thought, "you can't". But then I realised: Just the same as on your local host. That means, if you want perl to open your file, you will have to start a perl programme-or-script through your telnet session remotely.

This is a correct answer to your question, but I guess it isn't what you wanted? If you mean to open a remote file and process it locally, then telnet is just the wrong protocol*

Interactive terminal sessions and file transfer are different things. There tend to be protocols like ssh/scp/sftp or in the olden days telnet/ftp to address either issue.

* telnet is the wrong protocol in most situations. Unless it is the early 1990es again. Or we're stuck in ciscoland.

Cheers, Sören