...
%$hash_ref or return; # just return
...
$key;                 # or last line in sub returns a scalar

##</code><code>##

( key1 => ($val1), key2 => ($val2) )
( key1 => (),      key2 => ()      )
( key1 => ($val1), key2 => ()      )
( key1 => (),      key2 => ($val2) )

##</code><code>##

( 'key1', $val1, 'key2', $val2 )
( 'key1', 'key2'               )
( 'key1', $val1, 'key2'        )
( 'key1', 'key2', $val2        )