package Tie::SeqArray;

use strict;
use warnings;

use parent 'Tie::Array';

sub TIEARRAY {
    my $self = shift;
    my $class = ref($self) || $self;
    my $sequence_size = shift || die "sequence size of at least one required";

    return bless {seq_size => $sequence_size};    
}

sub FETCH {
    my $self = shift;
    my $i = shift;

    return $i >= 0 && $i < $self->{seq_size} ? $i +1 : undef;
}

sub FETCHSIZE {
    my $self = shift;

    return $self->{seq_size};
}

package main;

use strict;
use warnings;

# from Perl Cookbook http://docstore.mik.ua/orelly/perl/cookbook/ch02_18.htm
sub commify {
    my $text = reverse $_[0];
    $text =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1_/g;
    return scalar reverse $text;
}

my (@aa, @bb);
tie @aa, 'Tie::SeqArray', 20_000_000;
tie @bb, 'Tie::SeqArray', 20_000_000;

print $aa[100], ' ', commify(scalar(@aa)), "\n";
print commify($bb[19_999_001]), ' ', commify(scalar(@bb)), "\n";

__END__
101 20_000_000
19_999_002 20_000_000