use strict;
use warnings;

my $val = 'X';

my $min_len_subgroup;
sub chk {
  my $subgroup = shift;
  if(-1 != index($subgroup, $val)) {
    if (!defined $min_len_subgroup || length $subgroup < length $min_len_subgroup) {
      $min_len_subgroup = $subgroup;
    }
  }
}

our $re = qr{
  (
   \(
     (?:
       (?> [^()]+ )
          |
       (??{ $re })
     )*
   \)
  )
  (?{ chk($1) })
}x;

my @pats = (
  "(1(2(X)))",
  "(1(2(X,a))) (3(X,e,f))",
  "(1(2(X,b,c))) (3(X,f))",
  "(1(2(X,a))) (X,b)",
  "(1(2(X,a))) (X,b) X",
  "(1(2(X,a))) (X,b) (X)",
);

for my $pat (@pats) {
  undef $min_len_subgroup;
  if (() = ($pat =~ /$re/g)) {
    print "$min_len_subgroup\n";
  }
}