Code for alignment

#! perl
use strict;
use warnings;

my $MATCH    =  1;
my $MISMATCH = -1;
my $GAP      = -2;

my $seq1 = 'AAGAATAGTATTTCGCTTTTTTATA';

my $seq2 = 'AGAAATAGTATTTCGGTTAATTATA';

my @matrix;
$matrix[0][0]{score}   = 0;
$matrix[0][0]{pointer} = "none";
for(my $j = 1; $j <= length($seq1); $j++) {
    $matrix[0][$j]{score}   = $GAP * $j;
    $matrix[0][$j]{pointer} = "left";
}
for (my $i = 1; $i <= length($seq2); $i++) {
    $matrix[$i][0]{score}   = $GAP * $i;
    $matrix[$i][0]{pointer} = "up";
}

# fill
for(my $i = 1; $i <= length($seq2); $i++) {
    for(my $j = 1; $j <= length($seq1); $j++) {
        my ($diagonal_score, $left_score, $up_score);

        # calculate match score
        my $letter1 = substr($seq1, $j-1, 1);
        my $letter2 = substr($seq2, $i-1, 1);                         

        if ($letter1 eq $letter2) {
            $diagonal_score = $matrix[$i-1][$j-1]{score} + $MATCH;
        }
        else {
            $diagonal_score = $matrix[$i-1][$j-1]{score} + $MISMATCH;
        }

        # calculate gap scores
        $up_score   = $matrix[$i-1][$j]{score} + $GAP;
        $left_score = $matrix[$i][$j-1]{score} + $GAP;

        # choose best score
        if ($diagonal_score >= $up_score) {
            if ($diagonal_score >= $left_score) {
                $matrix[$i][$j]{score}   = $diagonal_score;
                $matrix[$i][$j]{pointer} = "diagonal";
            }
        else {
                $matrix[$i][$j]{score}   = $left_score;
                $matrix[$i][$j]{pointer} = "left";
            }
        } else {
            if ($up_score >= $left_score) {
                $matrix[$i][$j]{score}   = $up_score;
                $matrix[$i][$j]{pointer} = "up";
            }
            else {
                $matrix[$i][$j]{score}   = $left_score;
                $matrix[$i][$j]{pointer} = "left";
            }
        }
    }
}

1. trace-back

my $align1 = "";
my $align2 = "";

# start at last cell of matrix
my $j = length($seq1);
my $i = length($seq2);

while (1) {
    last if $matrix[$i][$j]{pointer} eq "none"; # ends at first cell of matrix

    if ($matrix[$i][$j]{pointer} eq "diagonal") {
        $align1 .= substr($seq1, $j-1, 1);
        $align2 .= substr($seq2, $i-1, 1);
        $i--;
        $j--;
    }
    elsif ($matrix[$i][$j]{pointer} eq "left") {
        $align1 .= substr($seq1, $j-1, 1);
        $align2 .= "-";
        $j--;
    }
    elsif ($matrix[$i][$j]{pointer} eq "up") {
        $align1 .= "-";
        $align2 .= substr($seq2, $i-1, 1);
        $i--;
    }    
}
$align1 = reverse $align1;
$align2 = reverse $align2;
print "$align1\n";
print "$align2\n";