The choice of a programming language is purely subjective, but when a student asks you which programming language he should start with, you have to make an answer, or at least provide some informations.
I think that a bioinformatician who studies R and at least two or three libraries (lattice/ggplot2, plyr) early can have an advantage, because he will be able to represent his data properly and obtain good results without too much effort. If your supervisor is not a computer scientist, he will be a lot more impressed by plots and charts than by programs, even if they are well written, with unittests etc.
Python is a good programming language to learn as a general purpose tool. Its bigger advantages are its easy to read syntax, and its paradigm 'there is only one way to do it', so the number of language keywords is reduced to the minimum, and two programs with the same function written by different people will be very similar (which is what doesn't happen with perl).
The negative points of python are that its CSV files reading/plotting interface is not ready yet (the best is pylab), so you must rely on R to produce nice plots.
Honestly I don't like perl, because I think it can induce to many bad-behaviours in novel programmers. For example, in perl there are many similar constructs to accomplish the same objective: so, it is very difficult to understand a program written by someone else, because you have to known all the possible constructs and hope there are enough comments. It is already very difficult to reproduce a bioinformatician experiment, if you write your code in a difficult language it is a lot worst.
Moreover, I know of many people who have been using perl for years, but that don't even use functions, because it looks too complicated. How can it be? It looks very inefficient.
The only good point of perl is its repositories, bioperl and CPAN; however, I know of people using perl that don't even know of the existence of these, so I don't understand why they keep going with perl.
Apart from programming language, is it very useful to learn the basic usage of gnu-make, or of a derivate. This program is very useful when you have lot of different scripts, as it allows you to define a pipeline in order to run them.
Some basic bash commands may also be very useful if you work with a lot of flat files (head, sed, gawk, grep, ...)