Using Modules and Exporter carefully in Perl

tl;dr : Playing Safe with Exporter

If you try to make a bunch of modules use each other and you are using Exporter to export variables accessed by these, you might run into the global symbol requires explicit package name error for those variables. In the following example, if you try perl -c Bar.pm you’ll get the said error for the %common symbol.

# Foo.pm
package Foo;
use strict;
use warnings;
require Exporter;
our (@EXPORT, @ISA);
@EXPORT = qw(%common);
@ISA = qw(Exporter);
use Bar;

our %common;
1;

# Bar.pm
use strict;
use warnings;
use Foo;

sub printstr { 
  print $common{somekey};
}
1;

Here, Bar is trying to use Foo to be able to access %common, so perl tries to compile Foo, but that in turn is trying to use Bar. This use statement gets compiled before %common has been exported and hence when compiler gets back to Bar it finds $common{somekey} and tells you that %common symbol requires explicit package name. Now, to make sure this doesn’t happen and the exported data is available without fail, you can wrap the exporter code in BEGIN {} so it gets evaluated at compile-time.

# Foo.pm
...
our (@EXPORT @ISA);
BEGIN { 
  require Exporter;
  @EXPORT = qw(%common);
  @ISA = qw(Exporter);
}
...

Although I did spend some time arriving at this when I was trying to improve the existing OpsBot code, thankfully its covered in the docs as well ! For information on how to use exporter in a better way, check out Good Practices with Exporter from the docs.