Nice idea.

A few minor nits though. First is that it is customary to only count the body of the sub, so you actually did slightly better than you thought! A second minor issue is that your grep is not quite right - the string "fooey\n" will match /^fooey$/ so you don't really have an equality test.

Further than that, there are some mechanical tricks that can be used to improve scores. Such as using an inline for loop, using map to avoid double loops, and the like. Using those I can take your answer to 75 fairly easily:

sub f{
#23456789_123456789_123456789_123456789_123456789_123456789_123456789_
+12345     
%h=@_;$h{$_}||=[]for map@$_,values%h;sort{-1+2*grep$_ eq$b,@{$h{$a}}}k
+eys%h     
}
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UPDATE
Oops. I didn't notice the lack of transitivity in your code. This is a fatal flaw as you see with the following data set.

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