#I have a hash,

%myhash;

#I pass it by reference:

mergetwohashes(\%myhash, \%b);

sub mergetwohashes {
    my($a)=$_[0];
    my($b)=$_[1];
    $a = { %{$a},  %{$b} };
}

#After I call the function, %myhash is unchanged. Why.