If an X server is available, it can be reached/detected via the %ENV hash:

if ($ENV{DISPLAY}) {
  print "Found X server at $ENV{DISPLAY}\n";
};
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Yes, I knew, but a random look at the pages I found with google seemed to suggest that this may be highly unreliable, in particular that it may remain set also where not available. I have quickly checked here on a few machines and I have the impression that it is indeed so.

Keep in mind that it might be practical if the UI-less variant can be used even when there is an X server available; examples are shell-scripts and pipes...

Indeed this is why I wrote

I would like to write a script that supports both a cmd line UI and a GUI (Tk), and in addition to cmd-line switches to activate one or the other[...]

In the first place.