sub get_exon {
    my $end=pop;
    my $begin=pop;
    # alias $_[0] to $chromosome to avoid copy semantics speed penalty
    for my $chromosome (@_) {
      return substr($chromosome, $begin - 1, $end - $begin + 1);
    }
}
# or perhaps 
sub get_exon {
    my ($begin,$end)=@_[1,2];
    # alias $_[0] to $chromosome to avoid copy semantics speed penalty
    for my $chromosome (@_) {
      return substr($chromosome, $begin - 1, $end - $begin + 1);
    }
}