Maybe you could clarify that technique a bit. I tried:

use Tie::Constrained;

# Other code as before, with and without SIG{__DIE__}

sub db_connect
{
    my (
        $dsn,       # data source name
        $error_msg  # if any
        );

    # Set up connection string...
    $dsn = "DBI:mysql:database=xxx;host=somehost;port=nn";

    tie $cfg::dbh,
        'Tie::Constrained' =>
                    {
                    test => sub {
                                    $_[0]->isa('DBI::db')
                                    and $_[0]->ping;
                                },
                    value => DBI->connect($dsn,
                                          "user", "passwd",
#'DBI:mysql:','','',
                             {RaiseError => 0, AutoCommit => 1}),
                    fail => sub {
                                    $_[1] = DBI->connect($dsn,
                                                     "user", "passwd",
                                 {RaiseError => 0, AutoCommit => 1});
                            if( $DBI::errstr)
                            {
                                # FAIL
                                $error_msg = "db_connect(): $DBI::errs
+tr";
                                print "$error_msg\n";
                                $cfg::db_retry_cnt++;
                                if( !$cfg::db_cxn_msg_sent &&
                                    $cfg::db_retry_cnt > 3 )
                                {
                                    print "would have sent email/sms\n
+";
                                    $cfg::db_cxn_msg_sent = 1;
                                }
                            }
                            else
                            {
                                # SUCCESS
                                $cfg::db_cxn_msg_sent = 0;
                                $cfg::db_retry_cnt = 0;
                            }
                                }
                    };
    $Tie::Constrained::STRICT = 1;
}
[download]

Didn't quite understand the DB def for 'value' being empty in example, but tried it anyway, got 'DBD::mysql::st execute failed: No database selected' at each execute, so tried filling in with real values as above, and got same errors as in orig post, ending with 'Can't call method "quote" on an undefined value at ./pd.pl line 83', then perl died.