sub f{
#23456789_123456789_123456789_123456789_123456789_123456789_123456789_12345     
%h=@_;$h{$_}||=[]for map@$_,values%h;sort{-1+2*grep$_ eq$b,@{$h{$a}}}keys%h     
}                                                                               

##</code><code>##

my @list = (
  a => ['b'],
  b => ['c'],
  c => ['d'],
  d => ['e'],
);