One easy way to do it is to use a hash:

my %tmp;
@tmp{@b} = ();
delete @tmp{@a};
@result = keys %tmp;
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But this assumes you don't care about the result being in the same order as @b. If @b is already sorted, you can get around that by throwing a sort in front of keys %tmp, though this is somewhat wasteful.

Update: Sheesh, I guess I'm a little slow at writing nodes. I swear those first two replies weren't there before :)