X25 is a rather old network protocol, and is a digital protocol to communicate over public lines (the phone company), like you would with ISDN or ADSL, although X.25 covers more ISO layers
There are multiple possible implementations:
1. Talk direct X.25 protocols with the other side.
2. Emulate TCP/IP over X.25.

In the second case, you can just use the perl network libraries to communicate.

In the iron ages, I used to setup X.25 on IBM/AIX, but I don't remember a lot of this, only that there were many parameters to configure.
If you really need a deeper insight in the protocol, you should go to a netwerk newsgroup.
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Dr. Mark Ceulemans
Senior Consultant
IT Masters, Belgium