Probably some permissions issue. Run ssh in verbose mode: 
$ssh = Net::OpenSSH->new($ZFS_HOST,
                         ctl_dir => $SSH_CTLDIR,
                         user => $SSH_USER, passwd => $SSH_PWD,
                         master_opts => '-vvv',
                         master_stderr_fh => \*LOG);

[download]

If that doesn't give you enough information about the cause of the problem, then, you can use truss to see what's happening at the OS level.

update: and BTW, you are not using the capture method correctly. Do it as follows: 

my ($stdout, $stderr) = $ssh->capture2({timeout => 20}, "$ZFS_CMD $typ
+e $UID $IDname $ou:");
$ssh->error and print LOG "Command $ZFS_CMD failed: " . $ssh->error;
print LOG "OUT=$stdout, ERR=$stderr, EXIT=$?\n";
print LOG "changeZFS() msg:$stdout\n";
return $stdout;

[download]

In reply to Re: Net::OpenSSH - connection from out a CGI script fails by salva
in thread Net::OpenSSH - connection from out a CGI script fails by tgdaero

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