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. :-)