so because i have no patience at all with regexes you can exploit the fact that your valid strings are always odd; they start and end with x and another x must be in the middle.

use strict;
use warnings;

while (<DATA>){
    chomp;
    # note the string IS always odd
    my $inter = int ((length $_) / 2)-1;
    my @char = $_=~/./g;
    if (scalar @char % 2 < 1){
        print "Not OK $_ (unbalanced)\n";
        next;
    }
    if (
          $char[0] eq $char[$inter+1] and
          $char[0] eq $char[-1] and
          $char[0] eq 'x'
        ){
      print "$_\t\tOK\n";
    }
    else {
       print "NOT OK $_\t[$char[0] $char[$inter+1] $char[-1]]\n";
    }

}

__DATA__
xxxxx
x1x2x...x
xxx
x.x.x
x12x..x
x123x...x
x123x.x.x
x12x1x

# out

xxxxx           OK
x1x2x...x               OK
xxx             OK
x.x.x           OK
x12x..x         OK
x123x...x               OK
x123x.x.x               OK
Not OK x12x1x (unbalanced)
[download]

L*

UPDATE: it can be semplified, or golfed, a lot using 5.010

use strict;
use warnings;
use 5.010;

while (<DATA>){
    chomp;
    # note the string IS always odd
    if ((length $_) % 2 < 1){
        print "Not OK $_ (unbalanced)\n";
        next;
    }
    if (($_=~/./g)[0,(int((length $_)/2)-1),-1]~~[qw(x x x)]){
      print "$_\t\tOK\n";
    }
    else {
       print "NOT OK $_\n";
    }

}
[download]

L*