When multiple indexes are provided as a hash index (as opposed to a hash slice), they are joined using $;. This is indeed a rarely used feature. (HoH are preferred.)

$ perl -wle'$h{ -extended => "no" } = 1; print for keys %h' | od -c
0000000   -   e   x   t   e   n   d   e   d 034   n   o  \n
0000015
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How is that syntax able to set an option?

%{ $RE{zip}{US} } is either magical (and the fetcher looks for /^-extended\034/) or it's populated with all the possible options.