I did not read the original problem statement :)

csv (in => $fh, out => undef, on_in => sub { ... }); supports streaming and does not store in memory (other than the current record. Rewriting my version to do that can be an exercise to the reader.

In preparation I found that PerlIO::via::gzip *only* supports open my $fh, ":via(gzip)", "file.gz"; and *not* open my $fh, ">", "file.gz"; binmode $fh, ":via(gzip)"; :( :(