Three things need to match:

• The name provided to use or require.
• The path to the file relative to a directory in @INC.
• The namespace provided to package.

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In your working version, you have

• use w;
• w.pm (relative to . in @INC)
• package w;

They match, so all's good.

This is what happens:

use w; loads w.pm, then checks if the w namespace provides a method named import. If it does, the method is called.

The module asks Exporter to provide this method and to export $var_in_w and sub_in_w when called (with no additional arguments). $var_in_w and sub_in_w are therefore exported.

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In your broken version, you have

• use A::w;
• A/w.pm (relative to . in @INC)
• package w;

The package directive doesn't match the other two, so problems are to be expected.

This is what happens:

use A::w; loads A/w.pm, then checks if the A::w namespace provides a method named import. If it does, the method is called.

The A::w namespace doesn't provide an import method. Nothing exports $var_in_w and sub_in_w.

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If the file is to remain at A/w.pm relative to the script's dir,

• Possible fix #1

  • -I . (buggy) or use FindBin qw( $RealBin ); use lib $RealBin; (correct)^[1]
  • use A::w;
  • A/w.pm (relative to . or $RealBin in @INC)
  • package A::w;

• Possible fix #2

  • -I A (buggy) or use FindBin qw( $RealBin ); use lib "$RealBin/A"; (correct)^[1]
  • use w;
  • w.pm (relative to A or $RealBin/A in @INC)
  • package w;

The choice of fix will come down to this question: Is the module named A::w (fix #1), or is it named w and happens to be in a directory named A (fix #2)?

For example, one might organize their project as follows:

• scripts/
  • script.pl
• lib/
  • Foo/
    • Bar.pm

This could be coded as follows:

scripts/script.pl

use FindBin qw( $RealBin );
use lib "$RealBin/../lib";

use Foo::Bar;

...
[download]

lib/Foo/Bar.pm

package Foo::Bar;

...
[download]

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1. $RealBin is the directory that contains the script. . is the current work directory, which has nothing to do with the directory that contains the script. If they happen to be the same, it's just a coincidence.

Update: Many tweaks to presentation.