This is a more computation-efficient way that takes advantage of the fact that hash lookups are essentially O(1):

my %listA;
@listA{@a}=();
foreach my $item ( @b ) {
    next if exists $listA{$item};
    push @c, $item;
}
[download]

Of course CPAN is your friend for this sort of thing. But I wanted to show the hash technique because it replaces a single slow O(n^2) algorithm with two faster O(n) operations, which is an order of magnitude more computationally efficient (but more of a memory hog too).