how to determine WAN IP address from behind a NAT router?

[A.Melon <juicy@melontraffickers.com>]

I have connected a Debian box attached to a NAT router to a cable
modem, and I'd like to determine (preferably from Perl but
anything will do!) my "internet" IP address (as opposed to
192.168...).

I searched for information on this and found only solutions by
parsing the output of websites that display the HTTP REMOTE_ADDR.
But I don't want to rely on an external website.

I know there must be a way to do this because Gnutella-type
programs (client) display my "outside" IP address.  How do they
do it?