# with a coderef
$pssh->add_host($host,
    user =>'root',
    passwd=>'access',
    on_error=> sub {
        my ($pssh, $label, $error, $task) = @_;

        $error == OSSH_JOIN_FAILED and do { ... };
        $error == OSSH_ABORTED     and do { ... };
        $error == OSSH_GOTO_FAILED and do { ... };
    },
);

# or with constants
$pssh->add_host($label, on_error => OSSH_ON_ERROR_IGNORE);
$pssh->add_host($label, on_error => OSSH_ON_ERROR_ABORT_ALL);