First of all, the returned hash is empty so it's no wonder that you the uninitialzed warning. I've never used Net::SNMP but from a quick glance at the docs I see:

$result = $session->get_request(
                          [-callback        => sub {},]     # non-bloc
+king
                          [-delay           => $seconds,]   # non-bloc
+king 
                          [-contextengineid => $engine_id,] # v3 
                          [-contextname     => $name,]      # v3
                          -varbindlist      => \@oids,
                       );
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so I think you're call of request is wrong and should read

$result = $session->get_request( -varbindlist => [$OID] );
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