That is, you can switch databases without having to reach into your code to change a statement.

I think batkins was getting at the same point that I'm about to make, but I'm not entirely sure, and I think there's some terminological disagreement, to boot. So to amplify a bit, what he's (probably) terming a symbolic reference, I think I'd call dynamic method selection, and it does, in fact, answer your concern (though it also occasionally gives me the heebie-jeebies). Since method lookup (and loading) are done at 'run-time' (an admittedly blurry distinction) you can do something like the following:

use XML::Simple ':strict';  # since example code doesn't have
                            # backward-compatibility issues
use DBX;
use strict;   # of course!
use warnings; 

my $config  = XMLin "config.xml";  # XML left as an exercise to the re
+ader
my $driver  = $config->{driver};
my $connstr = $config->{connection_string};
my $SQL     = $config->{query};

my $conn = DBX->$driver($connstr, "", "");

my $rs = $conn->query($SQL);

while($rs) {
    print $rs->field("client") . "\n";
    $rs->field("client", "test");
    $rs->move_next;
}
[download]

This does not, of course, prove that you can do anything with DBX that you can't do with DBI--to do that, I'd have to know something about DBX, and have used DBI more than twice. But it's all I have to offer, so I hope it sheds a little light. :-)

So to amplify a bit, what he's (probably) terming a symbolic reference, I think I'd call dynamic method selection, and it does, in fact, answer your concern.

Actually, that would raise a separate concern. Now, to protect myself against bogus stuff coming out of a config file, I have to wrap code in an eval block. And even then I'm not guaranteed that what comes out of the config file won't be the name of some method in DBX that I don't want to call.

Actually, the DBX package doesn't contain any methods. So you can ONLY use it to initiate a connection.