Slightly off-topic, but when writing such a templating engine, I prefer to deal with missing replacement values by leaving the template parameters as-is (that is, replacing the template parameter by itself instead of a (missing) value):

$string =~ s|(\@begin.*?\@end)
            |shift @replace // 'Missing parameter for ' . $1
            |sexg;
[download]

I find this helps me debug the template better, as I easily see which template expression failed. This still does not help me see why a value was missing, but at least I know where the value was missing.