For Q.2: In order to avoid race conditions (where you check fo r the record existence in the db, but someone else inserts it before you have a chance of inserting it) you should let the database do the checking.
This is normally done with a constraint (usually a unique index of some sort). You then trap that error in the insert logic. Something like this:

# Assumes RaiseError is set
eval {
  $dbh->do("insert ....");
};
if($@) {
    # check for the "duplicate insert" error here, and 
    # handle accordingly...
}
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Michael