Since use happens at compile time and both the if and else parts of your code get compiled, you proposed code has the same effect as:

use LayerB2;
use LayerB;
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which probably won't work very well.

You can get around that with:

BEGIN {
    if ($version == 2) {
        eval "use LayerB2";
    } else {
        eval "use LayerB";
    }
    die $@   if  $@;
}
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I'd actually suggest changing LayerB.pm to support both environments so that your staging environment is more likely to very closely match your production environment. Something like: use LayerB ( Version=>$version ); or use LayerB ( Production => 2!=$version );

        - tye (but my friends call me "Tye")