#!/usr/bin/perl

use strict;
use warnings;

my $var1 = "123x456x789xabc";
my $var2 = "";
my $vorkommen = 0;

($var2 = $var1) =~ s/^((?:.*?x){$vorkommen})[^x]*?(x.*)$/${1}PROBE$2/;

print $var2;