<nit>3-arg open was added in 5.005_03, not 5.6.</nit>
Really? If that's true, it seems to be undocumented (or obscurely documented). I was basing my statement off of perl56delta listing it as a core enhancement. And there's no reference to it in perlfunc in 5.005_03. Compare "open" in these two:
-xdg
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