csv (in => $fh, out => undef, on_in => sub { ... }); supports streaming and does not store in memory

Nice, good to know!

I found that PerlIO::via::gzip *only* supports open my $fh, ":via(gzip)", "file.gz";

Too bad, but IO::Compress::Gzip (core module) that I showed should support that AFAIK.