I'd go one step further and make a hash of the contents of @$b for lookup purposes. It's faster to look up values in a hash than in an array.

sub exclude {
   my ($a, $b) = @_;
   my %lookup = map { $_ => undef } @$b;
   for (1..@$a) {
      my $i = shift @$a;
      push(@$a, $i) unless exists $lookup{$i};
   }
   @$a; 
}
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