#!/usr/bin/env perl
use 5.010; use warnings; use strict;

my $DNA = "AACCGTTAATGGGCATCGATGCTATGCGAGCT";

sub make_rand_getter {
    my $s = shift;
    my $sl = length $s;
    return sub {
        return substr $s, int rand($sl - $_[0]), $_[0];
    }
}

my $rstring = make_rand_getter($DNA);
say "3-letter string: " . $rstring ->(3);
say "7-letter string: " . $rstring ->(7);