It's not precisely the same thing as you're asking for... but would word-boundary (\b) be sufficient?

If not, you can just re-nest the look-behind and the alternation into: 

$text =~ s/(^|(?<=\s))"(?=\w)/“/gs;

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That works. They key is that the ^ doesn't have to be look-behind, since it's zero-width, anyway. 
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:Wq
Not an editor command: Wq

[download]

In reply to Re: Match whitespace or start-of-string with lookbehind by etcshadow
in thread Match whitespace or start-of-string with lookbehind by BigLug

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