Since you're concerned about memory, you could do something like a Merge Sort.
Memory: O(1) (Not counting @a, @b and @c)
Speed: O(A+B) (Assuming @a and @b already sorted. As good as the other solutions) 

my @a = sort { $a <=> $b } (43,43,44);
my @b = sort { $a <=> $b } (43,43);

my @c;
while (@a && @b) {
   if ($a[0] < $b[0]) {
      push @c, shift @a;
   }
   elsif ($a[0] > $b[0]) {
      die "Bad data";
   }
   else {
      shift @a;
      shift @b;
   }
}

push @c, $_ for @a;
die "Bad data" if @b;

[download]

A trivial change makes it non-destructive.

Update: Fixed bug mentioned in replies. Tested.

In reply to Re: Difference arrays. by ikegami
in thread Difference arrays. by BrowserUk

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