############ VALIDATE USER DATA #############

my $err_msg = '';

# check valid date format
$err_msg .= "<p>Wrong date format '$date', use YYYY-MM-DD\n" 
    unless $date =~ m#(\d{4})\-(\d{2})\-(\d{2})#;

my $year  = $1 || '';
my $month = $2 || '';
my $day   = $3 || '';

# check year
$err_msg .= "<p>Invalid year '$year'\n" unless $year > 1970 and $year < 2100;

# check month
# blah

# check day
# blah

# check other input adding error messages to $err_msg

do { show_error_page($err_msg); exit } if $err_msg;

########### ONLY GET HERE IF INPUT VALID ##########

do_work()