First, use RaiseError since you are not consistent about checking errstr (and in fact are checking a $dbh errstr on a $sth call).

Second, USE PLACEHOLDERS! (I yell because it's been said so many times on perlmonks it's hard to believe you haven't seen it).

Third, read the DBI docs about the rows() method - it rarely does what you want with a select statement. Instead, test the return value of the execute statement (but read the DBI docs about what execute returns).

update and bind_columns should come before an execute, not after (my bad) (thought it's hard to tell what you are wanting to do with it). Also look up MySQL's replace command if you are going to be updating a row (no need to check first).

I really recommend you read the docs and some tutorials on DBI since your mistakes are DBI mistakes, not SQL or MySQL mistakes (in this case)