s{
  http://www\.something\.com/something # the URL you are looking for
                                       # remeber to backslash '.' 
  (?!\w)                               # a negative look-ahead assertion
                                       # do not allow a dirname of 'somethinglong'
  /?                                   # an optional slash
}                                      # end of search pattern
{                                      # replace with
  http://www.something.com/something/index.html
}gx