Why do these hashes have different contents?

gistfile1.pl

#!/usr/bin/env perl

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

sub return_void {
    return;
}

sub return_undef {
    return undef;
}

my %h1 = (a=>1, b=>return_void(), c=>3);
print Dumper \%h1;

my %h2 = (a=>1, b=>return_undef(), c=>3);
print Dumper \%h2;

__END__

Output is:
Odd number of elements in hash assignment at ./ptest.pl line 15.
$VAR1 = {
          'a' => 1,
          '3' => undef,
          'b' => 'c'
};
$VAR1 = {
          'c' => 3,
          'a' => 1,
          'b' => undef
        };