Track DB changes by saving the Hashed rows into a flat file and compare with queried Data next time

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I have a DB where I am not allowed to change the schema and the tables do not have last mod time in them.
I need to query this DB data daily and check if there are any rows that changed. I thought combining the columns and hashing it with one of the columns also as key of hash, and write the output to a flat file.
The next day, I would query the same DB and compare the new hash with a hash created by reading the flat file.

select hashbyte('md5', a+b+c), a from table1 where s='1';

I write the output to a file using Perl's Storable and read it using retreive.

What I found is that even if the columns b & c for a row has changed it does not recognize if there is any difference.

Any suggestions? Thanks in advance.