Are you trying to add them in the Net::OpenSSH::Parallel constructor (my $pssh = Net::OpenSSH::Parallel->new()) call? If so, take a look at the section on the 'add_host' method--specifically, it says, "[A]ny additional option will be passed verbatim to the Net::OpenSSH constructor later." So I suspect the following *MIGHT* work (*UNTESTED*):

    my $pssh = Net::OpenSSH::Parallel->new();
    $pssh->add_host($_, 
                    master_stderr_discard => 1, 
                    master_stdout_discard => 1, ) for @hosts;
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Hope that helps.