The thing that made this clear for me is realising that useing a standard Exporter-derived module absolutely must be done at compile time.

a my ($b, $c)
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doesn't compile, but

use A qw/a/;
a my ($b, $c);
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does (assuming package A exports subroutine a). How can the use statement affect compilation? It couldn't, without BEGIN {}!

The point is that Exporter->import defines a (probably via some clever version of eval 'sub ' . caller . '::' . $to_be_imported *) at compile time.

For an even subtler example of the nastiness that can occur, note that

my ( $b, $c );
a $b, $c;
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does compile, but not as you'd expect—the ‘indirect object’ syntax causes it to be compiled as

$b->a($c)
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* At least, I think that that's what happens; I got tired reading the Exporter source and didn't find out for sure. :-)