Learning Perl, 5th Edition

Chapter 17. Some Advanced Perl Techniques

What we’ve put in this book so far is the core of Perl, the part that every Perl user should understand. But there are a few other techniques that, while not obligatory, are still valuable tools to have in your toolbox. We’ve gathered the most important of those for this chapter.

Don’t be misled by the title of the chapter, though; the techniques here aren’t necessarily more difficult to understand than what we have elsewhere. They are “advanced” merely in the sense that they aren’t necessary for beginners. The first time you read this book, you may want to skip (or skim) this chapter, so you can get right to using Perl. Come back to it a month or two later, when you’re ready to get even more out of Perl. Consider this entire chapter a huge footnote.^[29]

Trapping Errors with eval

Sometimes, your ordinary, everyday code can cause a fatal error in your program. Each of these typical statements could crash a program:

$barney = $fred / $dino;         # divide-by-zero error?

print "match\n" if /^($wilma)/;  # illegal regular expression error?

open CAVEMAN, $fred              # user-generated error from die?
  or die "Can't open file '$fred' for input: $!";

You could go to some trouble to catch some of these, but it’s hard to get them all. (How could you check the string $wilma from that example to ensure that it makes a valid regular expression?) Fortunately, Perl provides a simple way to catch fatal errors—wrap the code in an eval block:

eval { $barney = $fred / $dino } ...