#!perl

use strict;
use warnings;

while (<DATA>) {
	chomp($_);
	print "$_ okay\n" if /^(\d)((??{$+ + 1}))+$/;
}

__DATA__
234
213
12345

##</code><code>##

234 okay
12345 okay

##</code><code>##

((??{$+ + 1}))+ matches null string many times
before HERE mark in regex
m/^(\d)((??{$+ + 1}))+ << HERE $/
at ./test.pl line 8.