Calling Compress::Raw::Zlib::deflate is fairly expensive.
Relying on the `.=' (concat) operator inside ->zadd operator is
faster, but the method dispatch overhead is noticeable compared
to the original code where we had bare `.=' littered throughout.
Fortunately, `print' and `say' with the PerlIO::scalar IO layer
appears to offer better performance without high method dispatch
overhead. This doesn't allow us to save as much memory as I
originally hoped, but does allow us to rely less on concat
operators in other places and just pass a list of args to
`print' and `say' as a appropriate.
This does reduce scratchpad use, however, allowing for large
memory savings, and we still ->deflate every single $eml.