Needleman-Wunsch perl script

[pony2001mx]

Dear all,
Actually i posted a similar question on Biostars about usage of Needleman perl module.
i have problem with the following script (kind helps from a biostars member). Could anyone help to correct it? THANKS A LOT!
My ideal output is:

CTAT-------CTA   CTATTTTTTTTCTG   -9

However, the following script produces the below result

CTATCTA   CTATTTTTTTTCTG   -9

script:

#!/usr/bin/env perl
use strict;
use warnings;
use Algorithm::NeedlemanWunsch;
### scoring function:
sub score_sub {
  if (!@_) {
    return -2;            # gap penalty
  }
  ## mismatch scores -1, match +1
  return ($_[0] eq $_[1]) ? 1 : -1;
}

my $a = "ATCTATC";
my $b = "GTCTTTTTTTTATC";
my @a = split //, $a; # make an array
my @b = split //, $b;
my ($a_align, $b_align);

sub on_align  {  
$a_align .= $a[$_[0]];
$b_align .= $b[$_[1]];
};
 
sub on_shift_a { $a_align .= $a[$_[0]];};
sub on_shift_b { $b_align .= $b[$_[0]];};

sub on_select_align { print "(select_align)\n"; return (keys (%{$_[0]})) [0]};

my $matcher = Algorithm::NeedlemanWunsch->new(\&score_sub);
my $score = $matcher->align(
                \@a,
                \@b,
                {   align     => \&on_align,
                shift_a => \&on_shift_a,
                shift_b => \&on_shift_b,
         # select_align => \&on_select_align
                });

print "$a_align\t$b_align\t";
print "$score\n";