Over on stackexchange there is a post on this very topic at tzset

That post asks

Notice how on my system, the hour changes without calling tzset. This holds true on recent versions of Perl in Ubuntu and Illumos, as well as Perl v5.8.8 on Solaris 10.

So if all my tests indicate that tzset has no effect, why / what other systems require tzset to be called explicitly? Do I still need to call tzset to remain compatible with certain environments, or is it now a thing of the past?

and one of the replies says

TL;DR: Starting with Perl v5.8.9 (released in 2011) calling tzset when changing $ENV{TZ} isn't needed anymore.

That seems to mean I could go into the code for SVG::TT::Graph and just edit out the call to tzset?

I can download the module from CPAN and I guess i'll find out if it is as easy as perl Makefile.PL in the downloaded module directory.. but.. then on Unix I'd do something like "make", "make test" and "make install". I guess I'll find out if the gmake is up to the job.