$sequence1 = [ qw( a b     ) ];
$sequence2 = [ qw(    b  c ) ]; 

# the longest common subsequence of it
$LCS_index = [[ 1, 1 ]];

# aligned
$result = [
  [ 'a', '' ],
  [ 'b', 'b' ],
  [ '',   'c' ],
];

$stringified = [
  'ab_',
  '_bc',
];