Since October 2010, Tcl 8.6 has had IPv6 socket support built in.
Ceptcl provides IPv6 among other protocols. Windows users might be interested in iocpsock.
Jim Tcl has had IPv6 and UDP support since Oct 2010.
tcllib_ip has (incomplete) support for parsing and manipulating IPv6 addresses.
Old (ca 2010, ~8.0, 8.3) Unix implementations might be able to use this patch: http://www.unnumbered.net/~umeno/etc/tclIPv6patch.htmkkl
See:
* [IPv6 Addresses] - a tiny module for parsing and formatting * [A Little 6to4 Calculator]
This [L1 ] thread hints at a couple of the challenges in getting Tcl's high-level socket programming interface right.
Stu 2009-06-17 I noticed that Ceptcl wasn't mentioned in that thread, I wonder why. I'd post this to the thread itself but my 'net conn's sporadic lately.