Just some side notes:

• Trace levels 1 and 2 are reserved for the DBI itself
• The DBI itself only features levels 1..15, but the underlying DBD might support up to whatever they like. I think I saw one that took all over 100 to be special, but my recollection might be troubled.
• Some DBD's support additional trace flags
  
  • DBD::Unify supports dbd_verbose (alias uni_verbose) 0..8
  • DBD::Oracle supports ora_verbose (range not specified)
  • DBD::Pg supports dbd_verbose passing the value to DBI
  • DBD::CSV documents support for dbd_verbose in the TODO list
  • DBD::Sybase, DBD::DBM, DBD::Firebird, DBD::SQLite, DBD::MariaDB, and DBD::mysql have neither
  • DBD::ODBC supports odbc_trace (true or false)
  (all that from the documentation, they might have undocumented debugging flags)
• Not all of the trace messages come from DBI itself, so check the DBD you use if they have additional documentation