Hidden Treasures of the Perl Core

The Perl Core comes with a lot of little modules to help you get thejob done. Many of these modules are not well-known. Even some of the well-known modules have some nice features that are often overlooked. In this article, we’ll dive into many of these hidden treasures of the Perl Core.


This module allows you to use MakeMakers to-be-installed version of a package. Most of the distributions on the CPAN conform to MakeMakers building techniques. If you are writing a Perl module that has a build system, then there would be a good chance MakeMaker is involved. Testing on the command line is common; I know I find myself doing it often. This is one of the places that blib comes in handy. When running my test suite (you all have test suites, right?) on the command line, I’m able to execute individual tests easily.

  perl -Mblib t/deepmagic.t

If you are building someone elses module and find yourself debugging a testing failure, then blib could be used the same way.


PC Load Letter, what the frell does that mean?! – Micheal Bolton

When pushed hard enough, the Perl interpreter can spew out hundreds of error messages. Some of them can be quite cryptic. Running the following code snippet under the warnings pragma yields the warning Unterminated <> operator at program.perl line 11.

  $i <<< $j;

Thankfully, diagnostics is an easy way to get a better explanation from Perl. Since we’re all running our important programs under the strict and warnings pragmas, it’s easy to add diagnostics to the mix.

  use strict;
  use warnings;
  use diagnostics;

The previous code snippet now yields the following warning:

  Unterminated <> operator at -e line 1 (#1)
    (F) The lexer saw a left-angle bracket in a place where it was expecting
    a term, so it's looking for the corresponding right-angle bracket, and
    not finding it.  Chances are you left some needed parentheses out
    earlier in the line, and you really meant a "less than".

  Uncaught exception from user code:
        Unterminated <> operator at program.perl line 11.

Use of the diagnostics pragma should be kept to development only (where it’s truly useful).


It can be difficult to benchmark code. When trying to optimise a program or routine, you want to try several approaches and see which comes out faster. That’s what the Benchmark module is for. This way, you don’t have to calculate start and stop times yourself, and in general you can do high-level profiling quickly. Here is an example that tries to determine which is faster, literal hash slices or retrieving hash values one at a time.

  use Benchmark;

  sub literal_slice {
    my %family = (
      Daughter => 'Evilina',
      Father => 'Casey',
      Mother => 'Chastity',
    my ($mom, $dad) = @family{qw[Mother Father]};

  sub one_at_a_time {
    my %family = (
      Daughter => 'Evelina',
      Father => 'Casey',
      Mother => 'Chastity',
    my $mom = $family{Mother};
    my $dad = $family{Father};

    5_000_000 => {
      slice       => \&literal_slice,
      one_at_time => \&one_at_a_time,

On the hardware I have at work, a dual G4 PowerMac, the answer seems obvious. Being cute and clever doesn’t hurt us too badly. Here is the output.

  Benchmark: timing 5000000 iterations of one_at_time, slice...
  one_at_time: 53 wallclock secs (53.63 usr +  0.00 sys = 53.63 CPU) 
         @ 93231.40/s (n=5000000)
        slice: 56 wallclock secs (56.72 usr +  0.00 sys = 56.72 CPU) 
         @ 88152.33/s (n=5000000)


Many of you know you can use Perl to write your HTML, in fact, this trick is often used in CGI programs. If you have used the CGI module to create HTML, then it would be obvious that the output is not intended for humans to parse. The ``browser only” nature of the output makes debugging nearly impossible.

  use CGI qw[:standard];

  print header,
    start_html( 'HTML from Perl' ),
    h2('Writiing HTML using Perl' ),
    p( 'Writing HTML with Perl is simple with the CGI module.' ),

The previous program produces the following incomprehensible output.

  Content-Type: text/html; charset=ISO-8859-1

  <?xml version="1.0" encoding="iso-8859-1"?>
  <!DOCTYPE html
          PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
  <html xmlns="http://www.w3.org/1999/xhtml"; lang="en-US">
  <head><title>HTML from Perl</title></head><body><h2>Writing 
  HTML using Perl</h2><hr /><p>Writing HTML with Perl is simple with the 
  CGI module.</p></body></html>

By changing the first line to use CGI::Pretty qw[:standard];, our output is now manageable.

  Content-Type: text/html; charset=ISO-8859-1

  <?xml version="1.0" encoding="iso-8859-1"?>
  <!DOCTYPE html
          PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
  <html xmlns="http://www.w3.org/1999/xhtml"; lang="en-US">
  <head><title>HTML from Perl</title>
          Writing HTML using Perl
          Writing HTML with Perl is simple with the CGI module.

While not as attractive as I’d like, there are lots of customizations to be made, all outlined in the CGI::Pretty documentation.


The world of class inheritance is a complex and twisting maze. This module provides some functions to help us navigate the maze. The most common need is for the function super_path(). When dealing with complex OO hierarchies, super_path() can help us know which classes we’re inheriting from (it isn’t always obvious), and find method declarations.

I have a little project that requires Class::DBI, so I ran super_path() on one of the classes to determine how Perl would search the inheritance tree for a method.

  perl -MJobSearch -MClass::ISA -le'print for 
      Class::ISA::super_path( "JobSearch::Job" )'

The following list of classes is in the order Perl would search to find a method.


Now if I have a question about a method implementation, or where methods are coming from, I have a nice list to look through. Class::ISA intentionally leaves out the current class (in this case JobSearch::Job), and UNIVERSAL.

Here is a little trick that allows me to find out which classes may implement the mk_accessors method.

  perl -MJobSearch -MClass::ISA -le \
    'for (Class::ISA::super_path( "JobSearch::Job" )) { 
        print if $_->can("mk_accessors") }'

Because of inheritance, all of the classes listed can invoke mk_accessors, but not all of them actually define mk_accessors. It still manages to narrow the list.

Class::ISA was introduced to the Perl Core in release 5.8.0. If you’re using an older Perl, you can download it from the CPAN.


This module makes it simple to find the current working directory. There is no need to go to the shell, as so many of us do. Instead, use Cwd.

  use Cwd;
  my $path = cwd;


Perl provides access to environment variables via the global %ENV hash. For many applications, this is fine. Other times it can get in the way. Enter the Env module. By default, this module will create global scalars for all the variables in your environment.

  use Env;
  print "$USER uses $SHELL";

Some variables are of better use as a list. You can alter the behavior of Env by specifying an import list.

  use Env qw[@PATH $USER];
  print "$USER's  path is @PATH";

Yet another module to save time and energy when writing programs.


This module has a useful function called mkpath. With mkpath you can create more than one level of directory at a time. In some cases, this could reduce a recursive function or a loop construct to a simple function call.

  use File::Path;
  mkpath "/usr/local/apache/htdocs/articles/2003";

Since mkpath will create any directory it needs to in order to finally create the 2003 directory, a tremendous amount of code is no longer needed.


This module implements a sane and useful interface over the File::Spec module. File::Spec must be used by calling class methods, while File::Spec::Functions turns those methods into functions. There are many functions that are all useful (and fully documented in File::Spec::Unix). Here are a few examples.

  use File::Spec::Functions qw[splitpath canonpath splitdir abs2rel];

  # split a path into logical pieces
  my ($volume, $dir_path, $file) = splitpath( $path );

  # clean up directory path
  $dir_path = canonpath $dir_path;

  # split the directories into a list
  my @dirs = splitdir $dir_path;

  # turn the full path into a relative path
  my $rel_path = abs2rel $path;

As you can see, there are plenty of ways to save yourself coding time by using File::Spec::Functions. Don’t forget, these functions are portable because they use different symantecs behind the senses for the operating system Perl is running on.


If you need a temporary file, then use File::Temp. This module will find a temporary directory that is suitable for the operating system Perl is running on and open a temporary file in that location. This is yet another example of the Perl Core saving you time.

  use File::Temp;
  my $fh = tempfile;

  print $fh "temp data";

This will open a temporary file for you and return the filehandle for you to write to. When your program exits, the temporary file will be deleted.


FindBin has a small but useful purpose: to find the original directory of the Perl script being run. When a program is invoked, it can be hard to determine this directory. If a program is calling chdir, then it can be even more difficult. FindBin makes it easy.

  use FindBin;
  my $program_dir = $FindBin::Bin;


Shell takes the ugliness of dealing with the command line and wraps it up in pretty functions. The effect here is prettier programs. Here is a simple demonstration.

  use Shell qw[ls du];
  use File::Spec::Functions qw[rel2abs];

  chomp( my @files = ls );
  foreach ( @files ) {
        print du "-sk", rel2abs $_;


This module allows localtime to return an object. The object gives you by-name access to the individual elements returned by localtime in list context. This doesn’t save us much coding time, but is can save us a trip to the documentation.

  use Time::localtime;
  my $time = localtime;
  print $time->year += 1900;

There is a similar module called Time::gmtime, which provides the same functionality for the gmtime function.


The UNIVERSAL module is handy. Two of its most common functions, isa and can are almost always used in OO programming as methods. isa is used to determine what class an object belongs to, and can will tell us whether an object supports a method. This is useful for testing. For example.

  use Time::localtime;
  my $time = localtime;

  if ( $time->isa( 'Time::localtime' ) ) {
    print "We have a Time::localtime object";

  if ( $time->can( "year" ) ) {
    print "We can get the year from our object";

Another less-known function in UNIVERSAL is VERSION. I often need to know the version of an installed module and I find myself writing a one-liner like:

  perl -MTest::More -le'print $Test::More::VERSION'

That’s just not as pretty as this.

  perl -MTest::More -le'print Test::More->VERSION'


The Perl Core has many hidden wonders, and I’ve just laid out a few here. Trolling the Core for interesting functions and modules has saved me a lot of work over the years. If you would like to look further, then browse the perlmodlib manpage for a list of the core modules. Whether your interest is CGI, I18N, Locale, or Math, you can find something there that saves a few hours of work.



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