Enabling tracing
 
 
- Per handle
- 
$h->trace($level);
 
 $h->trace($level, $filename);
 
- Only effects that handle and any new child handles created from it
- Child handles get trace level of parent in effect at time of creation
- Global (internal to application)
- Sets effective global default minimum trace level
- Global (external to application)
- 
- Enabled using DBI_TRACE environment variable
DBI_TRACE=digits		  DBI->trace(digits);
 
 DBI_TRACE=filename		  DBI->trace(2, filename);
 DBI_TRACE=digits=filename	  DBI->trace(digits, filename);