while ((my $begin, my $end) =  each %exon_endpoints) {
    print get_exon($chromosome, $begin, $end), "\n\n";
}

sub get_exon {
    my($chromosome, $begin, $end) = @_;

    # The arguments to substr are: string, beginning, length
    return substr($chromosome, $begin - 1, $end - $begin + 1);
}

##</code><code>##

while ((my $begin, my $end) =  each %exon_endpoints) {
    print substr($chromosome, $begin - 1, $end - $begin + 1), "\n\n";
}

##</code><code>##

my $chromosome;
get_chromosome(1, \$chromosome);

my $get_exon = sub {
    substr $chromosome, $_[0] - 1, $_[1] - $_[0] - 1;
};

while (my @ends = each %exon_endpoints)
{
    print $get_exon->(@ends), "\n\n";
}