The Net::SNMP module uses the procedure outline in Section 4 of RFC 3414 to perform discovery. Based upon the error you are seeing my best guess is the the APC device does not enforce timeliness checking and is not responding with a usmStatsNotInTimeWindows Report-PDU during the discovery process. You can work around this by editing the lib/Net/SNMP.pm at line 2545 and removing the ($this->{_error} =~ /usmStatsNotInTimeWindows/) part of the if statement.