That is the unexpected result that I gaet from the following code. Why is it making a hash?

Because this is what you wrote

my $hashref = {};
my %hash = $hashref;
[download]

%hash now contains a single key (keys are only strings), which is $hashref stringified (meaning no longer a reference). See for yourself

$ perl -MData::Dump -e " dd { {} } "
{ "HASH(0x3f8a6c)" => undef }
[download]

Say it with me:

my $hashref = { 'hashes', 'are', 'curly', 'ones' };

my $arrayref = [ 'arrays', 'are', 'square' ];

my %hash = ( 'lists are round' , 'hashes want pairs');

I highly recommend perlintro, http://learn.perl.org/books/beginning-perl/, http://perl-tutorial.org/, Modern Perl , Tutorials: Basic debugging checklist , brian's Guide to Solving Any Perl Problem