Catching errors with Perl DBI

How can I catch the errors generated whne I call an INSERT that violates a
constraint? I have coded like this:

             my $sth = $dbh->prepare($stmt);
             my $rv = $sth->execute() or die $DBI::errstr;
             if ( $rv < 0 ) {
                 print $DBI::errstr;
             }

But, if the INSERT violates a constraint, it never gets the the evaluation
of the $rv

Is this a setting for the DBI?

-- 
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
                        -- Benjamin Franklin