The infuriating thing was that it actually seems to work

Sure, it "works", but it's certainly not right:

$ perl -MData::Dumper -we 'my %hash = {}; print Dumper \%hash'
Reference found where even-sized list expected at -e line 1.
$VAR1 = {
          'HASH(0x814cc20)' => undef
        };
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It generates a warning and adds a useless entry to the hash. I can't imagine that's what the contractors wanted to do. :-)