The connection and authentication phases are carried out by the OpenSSH client and it doesn't pass the cause of the failure back to the module.

An easy to implement workaround is to redirect the output of the master ssh process to a file and then, to retrieve it when something goes wrong. 

my $ssh = Net::OpenSSH->new($host, master_stderr_file => "/tmp/ssh-std
+err-$host", master_opts => ['-v'], ...);

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In reply to Re^7: SSH2 - Asynchronous Opens & Synchronous Commands by salva
in thread SSH2 - Asynchronous Opens & Synchronous Commands by 5haun

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