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#!/usr/bin/perl

...
        delete $pairs{$vals[1]};
    }
}

[root@hadron ~]# ./t-1207429.pl t-1207429.txt
{"ip":"1.2.3.4","host":"www.example.com"}
{"ip":"2.3.4.5","host":"www.cnn.com"}
{"ip":"3.4.5.6","host":"www.google.com"}