sub mergedHashKeys{ 
    my( $r1, $r2 ) = @_;
    my %h = ( %{ $r1 }, %{ $r2 } ); 
    return [ keys %h ]; 
};;

$hash1 = {
    common_key => 'foo',
    hash1_specific_key => 'some value'
};;
$hash2 = {
    common_key => 'bar',
    hash2_specific_key => 'some other value'
};;

pp mergedHashKeys( $hash1, $hash2 );;
["hash2_specific_key", "common_key", "hash1_specific_key"]