# Get a line from the user (or file if redirected)
my $line = <STDIN>;
while ($line !~ /regex to check input/) {
    # input didn't match regex, so tell the user and get another line of data
    print "Sorry, I don't understand that, please enter the date like: DD-MM-YYYY\n";
    $line = <STDIN>;
}
... code that uses properly formatted date ...