# declare them here so 
# they are always in scope
my ($dbh, $sth);
eval {
    $dbh = DBI->connect(@connection_params, 
        {RaiseError => 1, 
         PrintError => 0,
         AutoCommit => 0}) || die "connection failed";
    # die-ing in the above line might seem 
    # redundant, since we are setting the 
    # RaiseError flag, but you don't know
    # for sure that was set, so best to be 
    # sure, since anything other than a valid
    # connection in $dbh is bad.
    $sth = $dbh->prepare($SQL);
    my $result = $sth->execute(@params);
    # ... do something with $result here ...

    # although it doesn't make sense to 
    # commit for a SELECT statement, if 
    # it wasn't a SELECT, i would commit
    # it here
    $dbh->commit();
};
# now lets check for an error/exception
if ($@) {
    # check for "our" error
    if ($@ =~ /^connection failed$/) {
        print "Database unavailable: " . $DBI::errstr;
    }
    # otherwise something in DBI went wrong
    else {
        print $DBI::errstr;
    }
    # again, transactions are irrelvant 
    # with SELECTs, but if you were to
    # need to rollback, you would do it
    # here.
    $dbh->rollback();
}
# clean everything up now
$sth->finish();
$dbh->disconnect();