my %hash = { };
[download]

I found this used several times in code we'd hired some contractors to do. The infuriating thing was that it actually seems to work...

You need to start using the -w flag more, my friend (that or use warnings). If you had, then perl would have warned you:

% perl -we 'my %hash = { }'               
Reference found where even-sized list expected at -e line 1.
[download]

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
        };
[download]

It generates a warning and adds a useless entry to the hash. I can't imagine that's what the contractors wanted to do. :-)