If you only want to change exactly http://www.something.com/something (with or without slash) to http://www.something.com/something/index.html try:

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