Earlier quoted context omitted.
Not sure if it would be considered simpler but lookaheads can be used to express it in a single pattern. ^(?=.{8}k.$)(?=[^a]*a[^a]*$)(?=[^b]*b[^b]*$)(?=[^c]*c[^c]*$)
It does lead to a very concise invocation: $ perl -ne 'print if /^(?=.{8}k.$)(?=[^a]*a[^a]*$)(?=[^b]*b[^b]*$)(?=[^c]*c[^c]*$)/' /usr/share/dict/words
[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