Found in /usr/share/perl/5.6.1/pod/perlfaq4.pod
       How do I compute the difference of two arrays?  How do I
       compute the intersection of two arrays?

               Use a hash.  Here's code to do both and more.  It
               assumes that each element is unique in a given
               array:

                   @union = @intersection = @difference = ();
                   %count = ();
                   foreach $element (@array1, @array2) { $count{$eleme
+nt}++ }
                   foreach $element (keys %count) {
                       push @union, $element;
                       push @{ $count{$element} > 1 ? \@intersection :
+ \@difference }, $element;
                   }

               Note that this is the symmetric difference, that
               is, all elements in either A or in B but not in
               both.  Think of it as an xor operation.
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"large inputs" tend to take a "long time" to process. Take that as a given in life -- something that's implied by the laws of physics

The only real caveat, is that sometimes when dealing with large inputs, the nature of the problem lends itself to a solution in which you can get intermediate results as you go, without having to first see all of the results. (ie: find all lines of input tha match a certain pattern).

Set operations do not fall into that category of problem -- you pretty much have to have both sets, in their entirety, available to you at the same time in order to find the difference (or intersection, or union)