A google search for X.25 comes up with lots of information. There are many variations of this protocol since it's been around for decades.

Are you using a hardware PAD or are you looking to develop a Perl software PAD using something like Device::SerialPort? How many circuits do you need to support? What circuit types are you using?

Update1: Search also for "IP over X.25". X.25 is a transport protocol and you will still need a higher level protocol to support the file transfer i.e. FTP. So, are you planning to tunnel IP via X.25? Cisco supports X.25 tunnels.

James