printf 'Certification is %s on %02d % 02d %d<p><HR>',
    $certification, $month, $day, $year;

##</code><code>##

for (my $i=1; $i<=$lines; $i++) {
	my $certification = shift(@data);
	my $number = shift (@data);
	my $month = $mon [$number-1];
	my $day = shift (@data);
	my $year = shift(@data);
	# printf ....
}

##</code><code>##

while(@data){
  ...
}

##</code><code>##

use Date::Calc qw/Month_to_Text/;
while(@data){
  ...
  my $number = shift (@data);
  my $month = Month_to_Text($number);
  ...
}