MST isn't unique; it could also mean Moscow Summer Time (according to the Olson files, this was used in 1917 only).

I'm not sure what your question is. I'm guessing that ActiveState's perl is using C library functions provided by Microsoft that interpret TZ as specified by the POSIX standard. If you have TZ=MST, it seems to default to a 0 offset, saying you are in a timezone named MST with offset 0. (When you say TZ=UTC+7, you are saying you are in a timezone named UTC with offset 7 hours. :)

Many unix-like systems have recognized the limitations of these POSIX-style timezone settings and switched to looking up the TZ environment variable in a database maintained by Arthur David Olson, thus allowing you to look up a timezone like MST (actually just a link to America/Phoenix; the Olson database names are usually in form (continent or ocean)/(major city) ). I wouldn't expect Microsoft to do so anytime soon.