DBD::CSV works fine for me with mod_perl. Here's an example script which will print "MAKING NEW CONNECTION..." on the first run and then "RE-USING EXISTING CONNECTION" on all the following runs:

#!/usr/bin/perl -wT
use strict;
print "Content-type: text/html\n\n";
my $dbh = My::DB->connect();
print "ok!" if $dbh->{RaiseError} == 1
           and $dbh->{f_dir} eq '/foo/bar';

package My::DB;
use strict;
use DBI;
sub connect {
    if (defined $My::DB::conn) {
        eval {$My::DB::conn->ping};
        print "RE-USING EXISTING CONNECTION ...<p>" if !$@;
        return $My::DB::conn if !$@;
    }
    print "MAKING NEW CONNECTION ...<p>";
    $My::DB::conn = DBI->connect(
        'DBI:CSV(RaiseError=1):f_dir=/foo/bar'
    );
    return $My::DB::conn;
}
1;
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Note, that it doesn't make any difference if the f_dir directory "/foo/bar" exists - no checking for existence of the f_dir is done at connection time, only when you attempt to access or create a table.

If the script above doesn't work for you, make sure that you are using the latest versions of DBD::CSV and DBD::File (which is found in the latest distribution of DBI).