With all the posts here you've pretty much got your answer if this will work or not. But I would also suggest making it readable. Make your code obvious what it is doing by making your variables self-explanatory and by commenting your code.

my %defaults = get_defaults();
my %bighash = get_big_hash();

# Set default values to our bighash
%bighash = (%bighash, %defaults);
[download]

The point here is to make that assignment statement as simple to read as possible so there is no confusion about what you are doing there and why.