Err, okay, my answer, short and sweet: use a hash and array slices:

my @a=(1 .. 2);
my @b=(2 .. 3);
my %c;
@c{@a,@b}=(@a,@b);
warn "Duplicates exist!" if scalar @c{@a,@b} != (@a+@b);
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Of course, in this instance you don't want the last line - instead you'd probably want something like:

my @d=keys %c;
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...which will be out-of-order, but guaranteed duplicate free.