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));
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This is in many ways superior because when C calls A you will have A completely compiled, if not yet executed.