Is there a way to tell IO::Socket::SSL to indicate to the server that it should use compression?

I don't know. I would think that, to begin with, both the openssl library against which IO::Socket::SSL (client) has been built, and the openssl library on the server would need to have been built with zlib support. Even then you'll probably have to rely on IO::Socket::SSL having been written with the capability of passing on the request for compression - which, one would hope, is documented (if such a capability exists).

I gather that ZLib is built into OpenSSL by default now

Best to check on that. When I built 0.9.8g a week or so ago, zlib support was certainly *not* the default for me - I had to explicitly declare zlib and -lz as ./config arguments.

When building openssl-0.9.8g you'll see lots of occurrences of -DZLIB during 'make' iff it's being built with zlib support. And during 'make test' you'll see numerous lines stating Available compression methods:. Iff zlib support was included it will be specified following each of those lines.

There are probably other ways to verify whether zlib compression was included, but I don't know what they are.

Cheers,
Rob