You will have to break the statement down into two parts:

results_table->{$$group}}->{$$site_name}->{sysobjid}="unknown;
results_table->{$$group}}->{$$site_name}->{snmp_status}="NOT OK";
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If you want to put the convenience back then use a fake loop to avoid the repetition:

for ($results_table->{$$group}{$$site_name})
{
  $_->{sysobjid} = 'unknown';
  $_->{snmp_status} = 'NOT OK';
}
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