What ever happened to Dumpx? It does not appear to be in that module.

A search of the git history of the Perl core seems to show that there never was a Dumpx function - only Dumpxs, so it might just be a typo in the book. Dumpxs is still there, but as choroba already pointed out, Data::Dumper defaults to the XS implementation (maybe it didn't in the past), so you normally don't have to worry about it.

$ perl -MData::Dumper -e 'print Data::Dumper->new(["foo"])->Dumpxs'
$VAR1 = 'foo';
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