you probably want a 4 byte string.

$ perl -le 'print pack("C*",split(/\./,"127.0.0.1"))' | od -t xC
0000000 7f 00 00 01
0000004
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which i think would match your snmpset 'x' "hex" example. you might also need to change the IPADDRESS to OCTETSTR or whatever Net::SNMP uses for a string of octets.