my @aWords;  # words from the abstract
my %hWords;  # maps words to their id
my $iUniqueWordsSoFar;  # cheap way to assign ids
my @aIds;    # sliding window with ids from last N-M words

for (0..($#aWords-$n)) {
   #look up/assign word id
   my $sWord = $aWords[$_];
   my $iWord;
   if (exists($hWords{$sWord})) {
     $iWord = $hWords{$sWord};
   } else {
     $iWord = $hWords{$sWord} = ++$iUniqueWordsSoFar;
   }

   # update sliding window of ids for last M words
   shift(@aIds) if scalar(@aIds);
   push @aIds, $iWord;

   # add key to hash for N..M length phrases by taking
   # first X elements of sliding window to construct 
   # the key.
}

# final pass: convert what is left in @aIds to keys and
# update appropriate phrase hashes.