The data you present is perfect for a hash (see perldata), since each row consists of a key (e.g. "politici") and a value (0.0489). You have two files with that structure, so you set up two hashes. You can then iterate over the keys of one hash with keys and see if it is present in the other hash with exists. Then access the corresponding values of both hashes and do your calculation.

for my $key (keys %left) {
    if ( exists $right{$key} ) {
        my result  = $left{$key} - $right{$key};
        print "$key | $result\n";
    }
}
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Storing the key/value tupels from the files into the hashes %left and %right is left as an exercise to the reader.