I agree that a systems programming language like C would complement Perl well; small overlap in their problem areas.

(Strictly commercially today, Java might be a better choice. I don't know.)

If you are going to learn C, I would recommend spending a few hours looking over processor architecture, if you don't know anything about it. C is quite close to the machine and it is enlightening to think about how C code would compile.

I am impressed by your will to learn. I know fewer languages after learning Perl! I stopped using AWK, shell and others and can't say I know them anymore. I know a better superset.

My favorite C book is Harbinson-Steele, but it is not that relevant anymore. Go with K&R or some other recommendation.