# Subroutine that takes a filename of the form DDMM.xxx and returns
# a filename of the form DDMMYYYY.xxx.

sub alt_name {
    local $_ = shift;
    die "Illegal filename" unless my ($dd, $mm, $ext) = /^(\d\d)(\d\d).(.+)$/;

    my ($day, $month, $year) = (localtime) [3 .. 5];
    $year += 1900;
    $month ++;

    if ($mm > $month || $mm == $month && $dd > $day) {
        # Assume file dates from last year.
        $year --;
    }

    sprintf "%02d%02d%04d.%s" => $dd, $mm, $year, $ext;
}