Modules in Perl are either Object Oriented, in which case they don't export and your code does not import any function, or they are procedural. In which case you use some of their functions as function.

If you use the default import of several procedural modules that all export many function, you might end up importing two different function, doing different things, but having the same name. In which case you will see a seemingly unpredictable behavior in which case the order of import will decide which function is in use.

Being explicit about what you import can greatly reduce this risk, and the maintenance programmer (you know, the proverbial psychopath, who knows where you live) will praise your name.

So how to make sure you don't forget to explicitly import functions?

As it turns out there is a Perl::Critic policy called Subroutines::ProhibitCallsToUndeclaredSubs that requires explicit importing of functions.

It is an ad-on policy that lives in the Perl-Critic-StricterSubs distribution, but after you install it, you can use it either on the command line or by enabling it in the .perlcriticrc of your project.

Here you can see it in action while using the single-policy flag: 

$ perlcritic --single-policy Subroutines::ProhibitCallsToUndeclaredSubs script.pl
Subroutine "greeting" is neither declared nor explicitly imported at line 5, column 1.  This might be a major bug.  (Severity: 4)

The code

The module we are using is a rather simple module that uses the import function of the standard Exporter and the @EXPORT array use by that import function to, well, export the 'greeting' function. This is one way to create a module for code reuse.

examples/explicit_import/MyModule.pm

package MyModule;
use strict;
use warnings;

use Exporter qw(import);
our @EXPORT = qw(greeting);

sub greeting {
    print "Hello World!\n";
}
1;

The script that uses the module, cleverly named 'script.pl', just loads the module and relies on the fact that the function is exported (and imported) implicitly.

examples/explicit_import/script.pl

use strict;
use warnings;

use MyModule;
greeting();

If you run this script, you will see it works: 

$  perl script.pl 
Hello World!

Explicit importing

While there is a certain level of danger of importing the same function name twice, I think the much bigger issue is the documentative value of importing functions explicitly. When 2 years after you wrote this someone needs to read and understand the code of the 'script.pl', which by that time grew to a 2,000 lines long monster using 15 different modules, it will be a great help to be able to easily locate the source of the 'greeting' function. The easiest way to that is to make the really little effort of explicitly importing the function.

examples/explicit_import/explicit.pl

use strict;
use warnings;

use MyModule qw(greeting);
greeting();

Of course, I know when you have to make some quick changes to a piece of code, and for some reason we are always asked to do it urgently, you will forget to do this. I certainly would.

In those cases it is a huge help to have a Perl::Critic policy configured and enabled in the tests that will remind me to do this 2-second long task.