Effective Perl free sample chapter: Files and Filehandles

Addison-Wesley converted our chapter on “Files and Filehandles” to HTML and put it online for as a free sample chapter. I selected this chapter as the free sample because it was the most fun to write but also the most valuable to new Perl programmers. Filehandles are the way you interact with the world, and using them wisely can give your program quite a bit of flexibility and make many tasks much easier.

Here’s the list of Items from that chapter, each of which you can read for free online:

We’ve also added more Items for “Files and Filehandles” in this blog, which you can also read for free. However, don’t forget about that Donate button on the right had side of the page if you find this site valuable. Or, buy our book and encourage all your friends to buy our book. Donations and book sales give us a little motivational boost to keep going. :)

Know what your the last evaluated expression actually is.

In Perl, a subroutine or other block structure that returns a value gives back the last evaluated expression, but if you’re not careful you might not recognize what that last evaluation actually is. It’s not necessarily the last statement in the block; it’s just the last one that you actually execute. For this Item, forget about the best practice of using explicit returns. You should do that for precisely the reasons you will see here, but you can’t learn about the problem by avoiding it. Continue reading “Know what your the last evaluated expression actually is.”

Use DBI_TRACE to follow DBI’s work.

There is a lot going on when you use DBI to interact with a database. Tack on a few layers of abstraction from Object Relational Modelers (ORM’s) such as DBIx::Class and you can end up with a tricky maze of subroutine calls to dig through when you need to track down issues. DBI comes with a built-in tracing feature to make it easier though. Continue reading “Use DBI_TRACE to follow DBI’s work.”

Effective Perl Programming’s table of contents

Here is the final table of contents for Effective Perl Programming, 2nd Edition. The “Item” references in our blog entries refer to the items in the book. We also have a map from the Item numbers in the first edition to those in the second, but we’ll have to do a little work to make those look nice for the blog. Continue reading “Effective Perl Programming’s table of contents”

Interact with the system when Perl isn’t enough

Usually, you want to do as much of your work inside your Perl program as you can. CPAN has a wealth of modules that accomplish many tasks, including those that you normally do very easily from the command line. In the cases where you can’t find a module, you might not be able to improve on the command-line siutation for complicated tasks. Perl is, after all, a glue language, so you don’t always have to do everything in Perl. When you decide to use an external program, there are many Perl features waiting to help, although you have to choose the right one for the job. Continue reading “Interact with the system when Perl isn’t enough”

Use /gc and \G in matches to separate alternations in separate, smaller patterns

Perl keeps track of the last position in a string where it had a successful global match (using the /g flag). You can access this position with the pos operator. With Perl 5.10, you can use the /p switch to get the per-match variable ${^MATCH} instead of the performance-dampening $&: Continue reading “Use /gc and \G in matches to separate alternations in separate, smaller patterns”

Know the difference between regex and match operator flags

The match and substitution operators, as well as regex quoting with qr//, use flags to signal certain behavior of the match or interpretation of the pattern. The flags that change the interpretation of the pattern are listed in the documentation for qr// in perlop (and maybe in other places in earlier versions of the documentation): Continue reading “Know the difference between regex and match operator flags”