use List::Util qw( max );

my $max_spam_len = max map length, 'Spam:', @spam;
my $format = "%-${max_spam_len}s  %s\n";

printf($format, 'Spam:', 'Ham:');

my $max_idx = max $#spam, $#ham;
for my $i (0..$max_idx) {
   printf($format,
      $i <= $#spam ? $spam[$i] : '',
      $i <= $#ham  ? $ham[$i]  : '',
   );
}