It's not clear to me what you're doing with $x -- are you using it as a symbolic ref? -- but the common way to construct a unique set is to use a hash. If $x were a hashref instead of an arrayref, you could simply use keys %$x to get the list -- though it would not be in original insertion order.

If you need the arrayref (or the ordering) you could keep a separate hash to track which values have already been inserted. That would look something like:

push @$x, $digit{$nu} unless $seen{$digit{$nu}};
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Or you could use Tie::Hash::Sorted to get the same effect from a hash.