dots in variable names on OS X

[davide zanon <d.zanon%infinito.it@localhost>]

Hi! sorry for bothering this list for the second time, but my messages
*never* get through... and I really can't find any info on the net.
I'm using Mac OS X 10.4.7, but I know this problem
exists on .4.2 too. Whichever shell I use, bash or tcsh (my
favourite) I cannot set any environment variable which contains
a dot in its name (eg PKG_OPTIONS.<package>), so I don't
have any control on pkgsrc build options. I only get:

tcsh: setenv: Syntax Error.

This is really annoying :)
Can anyone help me about this? Thanks a lot!

Davide