The only thing that comes to my head is that the multiplexing socket path could surpass the 108 chars of the sockaddr_un struture passed to bind.

You could try running ssh from the command line enabling the multiplexing feature with different paths, for instance:

ssh -x2M -S /tmp/mux localhost ls
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If that works, you could try with the exact same command executed by Net::OpenSSH. You will be able to see after enabling debug mode on your script:

$Net::OpenSSH::debug=-1;
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Besides that, the other thing that can be done is running ssh under the debugger in order to see what is being passed to bind.

I would do it myself, but have been unable to find any place offering free AIX accounts on the internet.