#!/usr/bin/perl -w

use strict;
use warnings;

main();

sub main {
        my %hash = ();
        $hash{'check'} = 'klappt';
        test(\%hash);
        print "2: $hash{'0'}\n";
}

sub test {
        my ($ref) = shift;
        my %hash = %$ref;
        print "dref: $hash{'check'}\n";
        $hash{'0'} = 'klappt nicht';
        print "1: $hash{'0'}\n";
}

print "2: $hash{'0'}\n";

Use of uninitialized value in concatenation (.) or string at ref.pl line 12.

sub test {
        my $href = shift;

        print "dref: $href->{'check'}\n";
        $href->{'0'} = 'klappt nicht';
        print "1: $href->{'0'}\n";
}