This won't work that way. You can only change the reference to point to another hash, but you cannot replace the hash itself. You could do this:

$myhash = \%myhash;

mergetwohashes($myhash, \%b);

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

But this will require you to only pass hash references, and work on the changed reference later. It will only change $myhsash, NOT %myhash.

Or you could do this:

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

But this way you only change the (complete) contents of the hash, the hash itself stays the same. But I'd guess this is just what you want...