Correct me if I'm wrong, but doesn't the  \K special escape emulate only the positive look-behind assertion? This is certainly suggested by perlre, in which the use of  \K for variable-length look-behind is discussed only in the context of the  (?<=pattern) assertion.

Update: However, the  (*SKIP)(*FAIL) pair of Special Backtracking Control Verbs (5.10+) can be pressed into service for negative look-behind:

>perl -wMstrict -le
"my $s = 'aaxbbbxccccxddxex';
 ;;
 printf qq{'$_' } for $s =~ m{ [a-e]+ x }xmsg;  print '';
 ;;
 printf qq{'$_' }
   for $s =~ m{ [bd]+ (*SKIP)(*FAIL) | [a-e]+ x }xmsg;  print '';
 ;;
 printf qq{'$_' }
   for $s =~ m{ (?: [bd]+ (*SKIP)(*FAIL))? [a-e]+ x }xmsg;  print '';
"
'aax' 'bbbx' 'ccccx' 'ddx' 'ex'
'aax' 'ccccx' 'ex'
'aax' 'ccccx' 'ex'
[download]