hLongest is empty, how can we find if exists here?

%hLongest is only empty when we read the very first line of the file. Thereafter it gets an entry for each and every list member we have found so far. How? Well, if you look at the line after the if (exists... statement, you'll see an assignment statement that creates an entry for the list member currently stored in $_. Or if an entry already exists, it updates it with the name of the longest list found so far.

The purpose of the if statement is to make sure that we only get to that assignment line, if the current list is longer than any other list containing the list member $_. First, it checks to see if we already have an entry for the list member $_ in the hash. That is the purpose of exists($hLongest{$_}) statement. If the entry is missing we go straight to the assignment.

If the entry isn't missing, then we look up the last list we found for the list member $_. If the list in the current line is shorter or the same size, then we skip to the top of the loop and start testing the next list member. next is what does the skipping for us. It insures that we never reach the assignment statement below it.

but in the result, even the shorter line which has sublist_153 gets added to result like this

That is happening because "mylist_1" is the longest line for "sublist_87" and "sublist_153" just happens to appear in both "mylist_12" and "mylist_1". This is part of the problem bart and Anno were trying to explain to you in the CB.

For some datasets, there is no way to satisfy the dual goal of (a) having only one line per list member and (b) having the longest list containing that list member.

Best, beth