Hello Anonymous Monk,

Similar question to yours was asked at the Monastery before How do I get the Nth Character of a String?.

Here are sample of codes from the relevant question:

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
use feature 'say';
# use Benchmark qw(:all) ; # WindowsOS
use Benchmark::Forking qw( timethese cmpthese ); # UnixOS

sub getn_unpack {
    return unpack "x" . ($_[1]-1) . "a", $_[0];
}

sub getn_substr {
    return substr $_[0], $_[1]-1, 1;
}

sub getn_split {
    return +(split //, $_[0])[$_[1]-1];
}

my $strNum = "12345678910";
my $string = "ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA";

# say getn_unpack($string, 10);
# say getn_substr($string, 10);
# say getn_split($string, 10);

my $results = timethese(1000000000, { 'unpack' => getn_unpack($string,
+ 10),
                      'substr' => getn_substr($string, 10),
                      'split' => getn_split($string, 10),
            }, 'none');
cmpthese( $results );

__END__

$ perl test.pl
              Rate unpack substr  split
unpack 171232877/s     --   -23%   -31%
substr 223713647/s    31%     --   -10%
split  248138958/s    45%    11%     --
[download]

It looks like the more efficient choice would be to use unpack. Something like that could do what you need. Reading one line at a time, extract the data that you want (one character) and finally push it into an array. Sample of code below:

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

sub getn_unpack {
    return unpack "x" . ($_[1]-1) . "a", $_[0];
}

my $file = 'data.txt';
my @array;

if (open(my $fh, '<', $file)) {
  while (<$fh>) {
      chomp;
      push @array, getn_unpack($_, 10);
  }
} else {
  warn "Could not open file '$file' $!\n";
}

print Dumper \@array;

__END__

$ cat data.txt
ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
ACATCACCTACCACAACGAGGACTACACCATCGTGGAACA

$ perl test.pl
$VAR1 = [
          'C',
          'A'
        ];
[download]

Update: Thanks to fellow Monk karlgoethebier for observing my mistake I would suggest an alternative solution to your problem. Use split instead of unpack. See sample of code below:

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

sub getn_split {
    return +(split //, $_[0])[$_[1]-1];
}

my $file = 'data.txt';
my @array;

if (open(my $fh, '<', $file)) {
  while (<$fh>) {
      chomp;
      push @array, getn_split($_, 10);
  }
} else {
  warn "Could not open file '$file' $!\n";
}

print Dumper \@array;

__END__

$ cat data.txt
ACATCACCTCCCACAACGAGGACTACACCATCGTGGAACA
ACATCACCTACCACAACGAGGACTACACCATCGTGGAACA

$ perl test.pl
$VAR1 = [
          'C',
          'A'
        ];
[download]

Hope this helps, BR