A few comments, naturally:

1. use strict warnings and diagnostics or die. These are your friends on the long run.
2. You declare 2 vars without need:

     my %h1 = %{ $_[0] }; #h2 similar
   [download]

3. Check whether your refs are defined:

     my %h1 = (defined $_[0] and ref $_[0]) ? %{ $_[0] } : ();
   [download]

   This will make the code more portable/general/cleaner. You may add an extra check for hashiness.
4. I just realized that this code can't distinguish between undefined or empty, so the grep becomes:

     scalar grep { 
       my ($a, $b) = ( $h1{$_}, $h2{$_} );
       ( not defined $a and defined $b )
         or
       ( not defined $b and defined $a )
         or
       $a ne $b 
     } @k1
   [download]

   Had to think extra carefully there!

Hope this helps,

Jeroen