#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my $q = 'ein_test_mit_zahl_123=456&ein_test_ohne_zahl=789';
# dies würde einen fehler erzeugen:
#$q .= '&ein_test=10';

print Dumper(resolve($q)), "\n";

sub resolve {
    my( $query ) = @_;

    my $res = my $tmp = undef;

    for ( split /\&/, $query ) {
        my( $path, $val ) = split /\=/;

        for ( split /\_/, $path ) {
            if ( /^\d+$/ ) {
                $$res = [] if ! defined $res;
                $tmp = $res if ! defined $tmp;
                $res = \$$res->[$_];
            } # if
            else {
                $$res = {} if ! defined $res;
                $tmp = $res if ! defined $tmp;
                $res = \$$res->{$_};
            } # else
        } # for
        
        if ( ref $$res ) {
            die "Incorrect query!\n'$path' points to a location that already holds data.\n";
        } # if
        else {
            $$res = $val;
        } # else
        
        $res = $tmp;
    } #for
    
    return $$res;
} # resolve