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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

  for (my $candidate = 10234; $candidate <= 98765; ++$candidate) {
    # we want to jump eg from 10992 to 11000
    # note not 'next': we've already incremented if the s/// succeeds
    redo if $candidate =~ s{ ^ (.*? (.) .*? \2) (.*) }{
      ($1 + 1) . ("0" x length($3))
    }ex;
    ...
  }

##</code><code>##

  print "$candidate\n" unless $bad;