you can play with length and more less greediness like in the following example (for sure while i'm writing you had received better answers)

use strict;
use warnings;

while (<DATA>){
    chomp;
    $_=~/x(.*?)x(.*)x$/;
    if (length $1 == length $2){
       print "OK $_\t [$1]",length $1," [$2]",length $2,"\n";
    }
    else{print "$_ NOT OK\t[$1]",length $1," [$2]",length $2,"\n";}

}

__DATA__
xxx
x.x.x
x12x..x
x123x...x
x123x.x.x
x12x1x

# out
OK xxx     []0 []0
OK x.x.x     [.]1 [.]1
OK x12x..x     [12]2 [..]2
OK x123x...x     [123]3 [...]3
OK x123x.x.x     [123]3 [.x.]3
x12x1x NOT OK    [12]2 [1]1
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