It is advisable as well to use strict; together with tachyon's comment about the use of values in the innner loop this leads to:

my (%authors, %subjects);
# and populate these hashes somehow

# the sort is irrelevant, at least for the part of the code you show
for my $from (keys %authors) {
   $authors{$from}->{$buddy}++
      for my $buddy map { split /\t/ } values %subjects;
}
[download]

and this seems rather strange because the values of %subjects never change, so each author ends up with exactly the same set of buddies. Is that what you intended? Or is some code missing/wrong in your post?

-- Hofmator