Another way to say it might be that the two-lines of the original post really do not give any concrete details as to, for example, what the input file actually looks like, and very little detail as to what the intended algorithm must be. Of course, bioinformatics is a very specific vertical within which the implied details of such a request might be “fully understood.” I would not know – it is not my vertical. It does seem to be yours, BrowserUK.
With regard to your particular reply, though, I would observe that it is extremely terse and devoid of comments. There is a lot going-on in that programming, in almost every single line, such that someone who also knows a lot about the Perl language could gaze in (justifiable) wonderment at it. However, anyone who knew less about Perl might find it difficult or even impossible to follow. The two statements, alone, which populate $dna in your example are stuffed with “golf.” I say that, not as any criticism of you, but rather, to point out why the result might well be “blank looks.” Knowing how to translate that example into the “key ideas” behind it might be even more difficult, according to one’s advance knowledge of Perl in-particular and programming in-general.
Since the OP says almost-nothing about what he wants, and nothing about what he is starting or ending with, and relies heavily upon the presumed-to-be mutually understood conventions of the bioinformatics vertical, this is a very tricky OP to respond to. (S)He also says nothing about the OP’s experience with the Perl language, hence ability to undestand and to make meaningful use of a highly-condensed example. Writing a “good” OP is always a bit of an art, and writing a response to “two-liner” is equally tricky. The responses in this thread seem to be a combination of good-intentioned requests for further clarification, and an (intrinsically) difficult-to-understand Perl code-example. Probably, we should shoot for a balance between the two.