BEGIN { use Exporter; ...

rir has asked for the wisdom of the Perl Monks concerning the following question:

In the last few years, I've started to notice code like this:

package Something;

BEGIN {
    use Exporter ();
    use vars qw($VERSION @ISA @EXPORT_OK);
    $VERSION     = '0.05';
    @ISA         = qw(Exporter);
    @EXPORT_OK   = qw(&errstr);
}
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To me, the BEGIN block around the Exporter stuff seems extraneous. What danger is this construction meant to avoid?

Be well,
rir

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Re: BEGIN { use Exporter; ... by tilly (Archbishop) on Dec 22, 2008 at 18:08 UTC
Some of it is cargo culting, but the marginal benefit comes if you have circular dependencies. If A uses B uses C uses A and you use A then when C calls A->import, A has only been compiled, not executed. This construct guarantees that the import will still work as expected. Avoiding the circular dependency is the better solution, of course. Update: My bad, I incorrectly started A's state when C calls A->import. At that point A has been compiled up to the `use B;` line and not executed. So you want the BEGIN block to be before that. The purpose of the construct is the same, though. An alternate solution is to have A: `require B; B->import(qw(some stuff if desired));` [download] This is in many ways superior because when C calls A you will have A completely compiled, if not yet executed.	[reply] [d/l] [select]

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Re: BEGIN { use Exporter; ...
by tilly (Archbishop) on Dec 22, 2008 at 18:08 UTC

Avoiding the circular dependency is the better solution, of course.

Update: My bad, I incorrectly started A's state when C calls A->import. At that point A has been compiled up to the use B; line and not executed. So you want the BEGIN block to be before that. The purpose of the construct is the same, though.

An alternate solution is to have A:

require B;
B->import(qw(some stuff if desired));
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