First we would have to answer WHY we would want to pack a hash (You probably meant 'pack' as in 'compress' or 'zip', right? The perl function pack has a different purpose than compression).

It would have the following advantage:

1. You could keep those few applications unchanged that are so big that they don't fit into memory anymore but that would still fit if compressed by about 5 to 50% (the usual compression rate of general compression algorithms, assuming you even could reach that with a hash compression, which is very doubtful). Most people would tell you to buy more RAM instead or adapt your algorithm/data structure

... and the following disadvantages

1. A compressed hash isn't a hash anymore, before accessing it you would have to uncompress it. You couldn't use $hash{'bla'} anymore. So you add a lot of inconvenience to the handling of hashes for the programmer

2. Uncompressing takes time. More than the time it takes to access a hash, so we are talking about a speed penalty of more than 100%. For a few free bytes more the hashes are seriously slower now.

3. Compressing algorithms don't guarantee compression. So even if you had a program that just needed 5% compression to fit into memory you never could be sure that your compressed hash could deliver it. With the "wrong" data your compression rate could be 0.

4.