Note that $hash->{username@domain}{EMLIST} is a reference to an array which you can access as @{$hash->{username@domain}{EMLIST}}. Therefore $hash->{username@domain}{COUNT} is very likely redundant because @{$hash->{username@domain}{EMLIST}} == $hash->{username@domain}{COUNT} and if it doesn't then there is most likely a bug in the code!

It's generally a bad idea to keep two copies of the same information because they are likely to get out of sync. Because it is trivial to find out how many elements are in an array keeping a separate count is redundant and error prone.