abowley@krait:~$ (echo 'Madrid(Spain' && echo 'Madrid(Spain)') | perl -pe 's/\((?![^)]*\))/ /g;'
Madrid Spain
Madrid(Spain)

##</code><code>##

s/
  \(      # an open bracket
  (?!     # _not_ followed by
    [^)]* # anything except brackets (e.g. 'Spain')
    \)    # followed by a bracket
  )
 / /gz;