Not exactly superman code, but it does what you want.

sub put_bracket {
  my $s = shift;
  my @m = @_;

  my @indexes = ();

  for my $match (@m) {
    my $lm = 0;
    while(1) {
      my $i = index($s,$match,$lm);
      last if($i eq "-1");
      push(@indexes,$i..($i+length($match)-1));
      $lm = $i+1;
    }
  }
  my %tmp = map { $_ => 1 } @indexes;
  my @s = sort {$a <=> $b} keys %tmp;
  my $n = '';
  my @chars = split(//,$s);
  my $marked = 0;
  for my $i (0..$#chars) {
    if($marked && $i == $s[0]) {
      shift @s;
    }
    elsif($i == $s[0]) {
      $n .= "[";
      shift @s;
      $marked = 1;
    }
    elsif($marked) {
      $n .= "]";
      $marked = 0;
    }
    $n .= $chars[$i];
  }
  return $n;
}
[download]