This problem needs something more than an AoA. Whenever you have a problem needing one of anything, it is best to start thinking "HASH". In this case we will need three hashes. While reading in data we will use two: one to keep track of the array associated with each list name ("mylist_1", "mylist_2", etc) and one to keep track of the name of the longest list found so far for each list member. Once we have the two hashes, we still have another step: finding only the list names that show up as longest lists. That will require using values and map as well as yet another hash.

The solution to this problem is a good illustration of three distinct, but very important uses of hashes:

• as an association between a name and a bunch of data (a string, a list, another hash, an object, etc)
• as a way of keeping track of the "most-est" - the longest, the smallest, the best, the worst, etc.
• as a way of generating unique lists from a list of possibly duplicate values

Here is the code. Please make sure you understand it. Feel free to post a reply if you have any questions: understanding how this works is far more important than the solution.

#always good to start your scripts with these two lines
#they let the compiler do about half of the bug catching
#and can save you lots of work

use strict;
use warnings;


#First we find which is the longest list for each list
#member

my %hLists;    #associate list name w/ list members
my %hLongest;  #keep track of longest list for each mbr.

while(my $line=<DATA>){
  #read in the list and its name
  my @aListMembers=split(/\s+/,$line);
  next unless scalar(@aListMembers); #skip empty lines

  #remember the name of the list
  my $sList = shift @aListMembers;
  $hLists{$sList} = \@aListMembers;

  # now for each member of the list record the name of
  # the longest list found so far
  my $iCount = scalar(@aListMembers);
  foreach (@aListMembers) {
    #we only care if its longer than what we already have
    if (exists($hLongest{$_})) {
      my $kLists = $hLongest{$_};
      my $aList = $hLists{$kLists};
      next if ($iCount <= scalar(@$aList));
    }
    $hLongest{$_} = $sList;
  }
}

#now we only care about lists that show up as the
#longest list for at least one element so lets get
#a unique "list" of those. We do that by creating a hash
#whose keys are the thing we want to be unique.  Then
#when we want the "unique list" we extract the keys of
#the hash.

my %hListsWeCareAbout = map { $_ => 1 } values %hLongest;

#now lets print out the results:
foreach (sort keys %hListsWeCareAbout) {
  my $aList = $hLists{$_};
  print "$_: @$aList\n";
}

__DATA__

mylist_1  sublist_153  sublist_87  sublist_876  sublist_78
mylist_6  sublist_8
mylist_2  sublist_12  sublist_34  sublist_09
mylist_3  sublist_87  sublist_09
mylist_7  sublist_8  sublist_9
mylist_9  sublist_56
[download]

Best, beth

Update: clarification of terminology, cleaned up some misuses of the word "list". Added sort to final print out. Added reminder about understanding the code. Added comments about how this problem illustrates 3 important uses of hashes.

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