#!/usr/bin/perl

use warnings;
use strict;

my $a = <<HERE;
<xml>
  <knoten>
    <2>!!!</2>
    <y>15</y>
  </knoten>
  <knoten>
    <4>!!!</4>
    <y>30</y>
  </knoten>
</xml>
HERE

my @b = split("\n", $a);

my @xvals = (2, 4, 6, 8);
my @collection = ();
my $i; my $u;
for ($i=0; $i < $#b; $i++) {
    foreach $u (@xvals) {
        if ($b[$i] =~ /<$u>[^>]*<\/$u>/) {
            if ($b[$i + 1] =~ /<.+>([^>]*)<\/.+>/) {
                my $href = {$u => $1};
                push(@collection, $href);
                last;
            } else {
                die "Error: No y-line after x-line,";
           }
        }
    }
}
my %result = ();

foreach $i (@xvals) {
    foreach $u (@collection) {
        my %hash = %{$u};
        my @onehashkey = keys(%hash);
        my $key = $onehashkey[0];
        if ($key eq $i) {
            $result{$key} += $hash{$key};
        }
    }
}

foreach $i (sort(keys(%result))) {
    print "$i => $result{$i}\n";
}