I would probably use something like:

while (ask('databasetype','Database type?','mysql')) {
    last
        if (grep {getConf('databasetype') eq $_} @databases);

    output "Sorry, invalid option\n\n";
}
[download]

And the ask() would return true if the user replied to the question and false if he accepted the default (no need to validate the default I guess) or pressed CTRL+D/CTRL+Z (end of input).

Jenda