What is the meaning/purpose of code like

*z=\$x;
[download]

it's... ehm, aliasing variables.

Is *var called the star sigil? Google doesn't seem to think so. Is this star stuff an artefact of an earlier version of perl?

Well, there are still cases in which it is useful to use typeglobs, but let's say that there are less than there used to be. As noted by someone else they won't be there in Perl6, but OTOH it will provide syntactical sugar to do (different kinds of) aliasing differently.

If there are still reasons to use *var with modern perl, where can I learn more about it?

Personally I think that it's useful to know about them, but if "there are reasons to" is up to you, as with a lot of other things in Perl. To do so I suggest perldata and the other documents referenced there.