The /e is the best option, I think.
But just to mention it, there is another way to do it: 
$b =~ s/\:\:/${\(File::Spec->rootdir())}/;

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The trick is to use de-reference a refence to a value. You can put almost any expresion inside the ${\( )} and it will be well evaluated.

In reply to Re^2: Function call inside regexp by nicoaimetti
in thread Function call inside regexp by jeanluca

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