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  for my $i (0 .. $#digits) {
    for my $j (0 .. $#digits) {
...
      ++$flag if $digit[$i] == $digit[$j];
    }
  }

  for my $i (0 .. $#digits) {
    for my $j ($i + 1 .. $#digits) {
      ++$flag if $digits[$i] == $digits[$j];
    }
  }

  sub check_duplicates {
    my $candidate = shift;
    ++$flag if $candidate =~ /(.).*\1/;
  }

  sub check_prime {
    my $candidate = shift;
    ++$flag if 2 != $candidate =~ tr/2357//;
  }

  for (my $candidate = 10234; $candidate <= 98765; ++$candidate) {
    # we want to jump eg from 10992 to 11000
...
    }ex;
    ...
  }

  print "$candidate\n" unless $bad;