I'm confused by your post. Breaking down qr/Foo::Bar(?!\s+\()/ we get:

  Foo::Bar  # literal 'Foo::Bar'
  (?!       # start of negative look-ahead
    \s+     # one or more white space chars
    \(      # literal opening bracket
  )         # end of negative look-ahead
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so to answer your reply, I believe I provided the literal ( after the \s*.

To answer your other statement, that something such as "Foo::Bar is my favourite module" should match, I agree - that is how I read the initial requirement from the thread. In which case my answer of using \s+ kept within the spirit of the original statements made by the thread author. However I wanted to keep my example in line with the example that the author had tried, hence the use of the literal (. Hope this clears up my answer.