[me@fedora ~]$ time perl -ne 'print if /^(?=.{8}k.$)(?=[^a]*a[^a]*$)(?=[^b]*b[^b]*$)(?=[^c]*c[^c]*$)/' /usr/share/dict/words | wc -l
  17

  real 0m0,250s
  user 0m0,245s
  sys 0m0,006s

    perl -ne '/^(?=([abc].*){3})(?!.*([abc]).*\2).{8}k.$/&&print' /usr/share/dict/words

  [me@fedora ~]$ time perl -ne '/^(?=([abc].*){3})(?!.*([abc]).*\2).{8}k.$/&&print' /usr/share/dict/words | wc -l
  6

  real 0m0,118s
  user 0m0,112s
  sys 0m0,007s