#!/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'
        ];