I've simplified the situation somewhat: it didn't crash per se, the error was reported by a die handler that does a somewhat orderly shutdown of the application in case of a fatal error.

The other answer in the thread has pointed out that the fix is as simple as removing the SSL_ prefix, so I'll report that to IO::Socket::SSL.

It's also worth noting that the error only appears if tracing is enabled, and we've had $Net::SSLeay::trace = 3 in our code. Removing this line also eliminated the errors. This might explain why this wasn't noticed by others until now: apparently it's in a rarely executed part of the code.