Like others have said, you have to trust some programmer, somewhere, at least a little bit. The nice thing about Open Source, Perl and Cpan, is that it is "all laid out for everyone to see", and word spreads pretty fast if there are major problems with something. The opposite is closed source like Microsoft, where nothing is allowed to be seen, and viruses and trojans spread very fast. You have to be allowed to look at the source, even if you can't understand it all. I worry more about the underlying C libraries, where it is almost total trust, than I do Cpan modules.
Safe, foolproof programs are like "The Holy Grail".... always to be sought after, but never found.
I'm not really a human, but I play one on earth.