{
    my %data;
    @data{@data} = 1;  # turn @data into a lookup hash
    foreach ( @data2 ) {
        print "This $_ was not found.\n"
            if not exists $data{$_}
    }
}
[download]

Update: An aside with MeowChow has convinced me that     @data{@data} = undef;  # turn @data into a lookup hash is less misleading. As with the "= 1" form above, only the value for the first key is set, and all remaining values are set to undef. Since we're just using the hash as a way to trade time for space in detecting the presense of keys, values don't matter. But the originally was slightly misleading nonetheless.