Mastering Perl, 2nd Edition

Chapter 17. Modules as Programs

Perl has excellent tools for creating, testing, and distributing modules. Perl’s also good for writing standalone programs that don’t need anything else to be useful, but we don’t have tools for standalone programs as good (or at all) as those for modules. I want my programs to use the module development tools and be testable in the same way as modules. To do this, I restructure my programs to turn them into modulinos.

The main Thing

Other languages aren’t as DWIM (Do What I Mean) as Perl, and they make us create a top-level subroutine that serves as the starting point for the application. In C or Java, I have to name this subroutine main:

/* hello_world.c */

#include <stdio.h>

int main ( void ) {
    printf( "Hello C World!\n" );

    return 0;
    }

Perl, in its desire to be helpful, already knows this and does it for me. My entire program is the main routine, which is how Perl ends up with the default package main. When I run my Perl program, Perl starts to compile the code it contains as if I had wrapped my main subroutine around the entire file.

In a module most of the code is in methods or subroutines, so, unlike a program, most of it doesn’t immediately execute. I have to call a subroutine to make something happen. Try that with your favorite module; run it from the command line. In most cases, you won’t see anything happen. I can use perldoc’s -l switch to locate the actual module file:

% perldoc -l Astro::MoonPhase /perls/perl-5.18.0/lib/site_perl/5.18.0/Astro/Sunrise.pm ...