I was expecting missing_ids_list to have less elements than idslist since the loop goes only through this number.

I think you may have missed the original premise in the thread which you referenced which was:

Given an arbitrarily long list of numbers that SHOULD be consecutive, is there a simple method of finding any missing numbers?

So, given a list like

my @idslist = (100,200,300);

you can see that the number of missing IDs exceeds the number in @idslist.

The code snippet you have quoted (from here?) works by pushing ascending lists of the values between consecutive items of @idslist onto the missing list. If the values in @idslist are not consecutive (or not ascending) then nothing is pushed. Hope that explains it for you.