From my experience, one of the most important reasons to use "language X" from "language Y" is to reuse code. Often you already have some well-tested, stable code - and it is much easier to create a wrapper for that code than to re-invent it.

If you are considering using different programming-languages (e.g. Perl and C) in a new project, do a good analysis if the advantages really outweight the (potential) troubles ... according to my experiences (as well as to the statements from ww and lanX in this thread) this is very doubtful.

HTH, Rata