I originally learned what Perl I know back in the 90's, and tended then (and still do, sometimes) to put common subroutines into a 'standard.pl' library which I would 'require' where needed. I know that most people bundle such routines into a module these days, and I understand some of the reasons for this. I was burned today and wondered if someone could explain to me (using small words) just why.

Suppose I have the following modules:

package A;
use B;
require 'standard.pl';
sub new {
#  [snip]
  bless ($self, $class);
  do_something(); # subroutine in standard.pl
  return $self;
}

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package B;
require 'standard.pl';
sub new {
#  [snip]
  bless ($self, $class);
  do_something(); # subroutine in standard.pl
  return $self;
}

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# standard.pl
# [snip]
sub do_something {
  print "Howdy.\n";
}
1;

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When I tried to run a script that used module A, I got an error message like this:

Undefined subroutine A::do_something called at A.pm line X.

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I eventually realized that module B was requiring the same standard.pl library and changed the code in A to no longer require the standard.pl libary and the invocation of the do_something() routine from:

  do_something();

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to:

  B::do_something();

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... which worked. It made me feel a little deceptive, referring to a common subroutine in standard.pl as though it was part of module B, though. Why couldn't module A 'see' that do_something() subroutine in its own namespace, since I explicitly 'require'-d it in both modules?

Maybe the moral of the story is that I shouldn't be mixing packages and libraries, but I would like to understand this better in any case.

In reply to Burned by 'require'? by ptum

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